Athlete comebacks are not wholly uncommon, they occur in an array of sports with athletes often simply changing their mind about their earlier decisions to end their sporting careers. The comebacks on this list, however, are slightly different. These athletes all came back in spectacular fashion after overcoming a serious mental and/or physical hurdle.

From career and life-threatening injuries and illnesses to devastating bouts of depression and substance abuse, all these athletes hit rock bottom, only to spectacularly overcome their problems and difficulties to return to their sport. Many of these stories are truly incredible and awe-inspiring, worthy of epic movies and fascinating books. In fact, a number of them have actually been turned into films.

With sporting comebacks being a fairly regular occurrence, many athletes have had to deal with very difficult events and issues to return to their sport. As a result, a number of admirable and great comebacks do not make this list, such as those of Michael Jordan, Tony Conigliaro and more. The list intends to take into account both the magnitude of the issue or issues faced by the athlete as well as their achievements upon their return. Here are the top 20 athletes who came back after hitting rock bottom:

20 20. George Foreman

George Foreman just sneaks into this list by virtue of his incredible achievements following his comeback. Foreman destroyed everything in his wake early in his career, before becoming unstuck against Muhammad Ali in the Rumble in the Jungle. Following his second defeat to Jimmy Young, Foreman believed he suffered a near death experience. He became a born-again Christian and did not take to the ring again for 10 years, serving as an ordained minister during this time. When he returned, in 1987, he was 38 years of age and badly out of shape. Seven years later though, aged 45, Foreman stunned the world of boxing by regaining the world heavyweight title in one of the greatest sporting comebacks of all time.

19 19. Jonah Lomu

Recently deceased rugby legend Jonah Lomu sadly passed away in November 2015, aged 40, and the rugby world mourned the passing of the sports first true worldwide superstar. Lomu became the youngest ever All Black, aged 19, and was considered the finest player at the 1995 Rugby World Cup, aged only 20. A year later he was diagnosed with a rare and serious kidney disorder which would trouble him for years to come. Lomu soon returned though, and battled on until 2002, when it was finally decided he needed a kidney transplant, and he underwent the transplant in 2004. He surprisingly returned to the sport in 2005, but was unable to break back into the All Blacks team before his full retirement from rugby in 2007.

18 18. Andre Agassi

One of the greatest tennis players of all time when he was at his best, Andre Agassi is a legend of the sport and largely attributed with the rise in the sports popularity during the 1990s. Born in Las Vegas but of Armenian descent, Agassi won a total of 8 Grand Slams over the course of his career. Ranked as world number one in 1995, Agassi had a dramatic fall from grace. Injuries, off court issues and a reported loss of love for the sport saw him plummet to world number 140 by 1997, by which time he was using crystal meth. In 1998, Agassi went clean, and set the record for the highest growth by a male player in a calendar year, moving from 140th to 6th. In 2002, he ranked as world number 2, the oldest player to ever rank that high.

17 17. Jim Morris

The story of Jim Morris was so 'magical' that Disney even turned it into a film, entitled 'The Rookie'. A highly promising baseball player in the minor leagues, a recurring arm injury prevented Morris from making it as a pro. He was drafted by the New York Yankees but was never signed, and eventually gave up on his dreams of making it in MLB in his early 20s, becoming a sports science teacher and baseball coach. Having promised his team that he would try out for MLB if they won the district championship, he did just that. The 35-year-old threw 12 98 mph pitches and was signed by the Tampa Bay Devil Rays. His MLB career lasted only 8 months but was an extraordinary story and Morris is now a well-paid after dinner speaker.

16 16. Carson Palmer

Arizona Cardinals quarterback Carson Palmer hasn't had it easy throughout his career. The three-time Pro Bowl selection underwent serious rehabilitation in January 2006 having suffered a torn ACL and a torn MCL. To make matters worse, Palmer had also dislocated his knee, sustaining serious cartilage damage in the process. Any of these injuries in isolation are potential career enders, when combined, it is a minor miracle Palmer returned to football, especially so speedily. Remarkably, he returned in August 2006, throwing more than 4,000 running yards the following season, with a quarterback rating of 93.9%, picking up the Pro Bowl MVP in 2006.

15 15. Muhammad Ali

There will be mixed views on Muhammad Ali's near four-year break from boxing following his refusal to conscript and join the war in Vietnam. Criticised by many, to some it only added to the legendary status of Ali. The three-time world champion was stripped of his titles (except for in the UK), and did not box for over three and a half years. The years Ali missed were between his 25th and 29th birthday, arguably some of his finest years as a boxer. Most people maintain Ali was best before the suspension, a period in which he went undefeated, dazzling opponents with his lightning speed. After his comeback though, Ali may have lost some of his speed, but none of his genius or bravado. He won back the world heavyweight title on his return in 1970 and fought his last bout in 1981.

14 14. Josh Hamilton

Few players in the history of baseball have shown such promise from an early age as Josh Hamilton. Touted for greatness right from his early high school days, the youngster was a first round first draft pick by the Tampa Bay Devil Rays, who he joined with a sign-on bonus just shy of $4 million. His growing reputation continued after his draft in 1999 right up to a car crash in 2001. At this time, Hamilton's career began to unravel. He started experimenting heavily with alcohol and drugs beginning a downward spiral which would ultimately see him out of the game for more than two years, cover himself in satanic tattoos and try to OD numerous times. The now 34-year-old Texas Rangers outfielder came back from rock bottom though, and is currently a five-time MLB All-Star.

13 13. James J. Braddock

James J. Braddock's career is a quite remarkable one. There can only be few world champions in any division, never mind the heavyweight division, to have a record of 50 wins, 26 defeats and 7 draws. Braddock showed promise early on, going 25 bouts undefeated, but had a horrific run in the late 1920s and early 1930s, losing 20 out of 29 fights at that stage of his career, despite being in his mid 20s at this time. His woeful results and performances, largely attributed to a series of serious hand injuries, appeared to have spelt the end of a once promising career. Braddock hadn't read the script though, and between 1933 and his retirement in 1938, he lost only once, and became world heavyweight champion between 1935 and 1937.

12 12. Frank Mir

A two-time UFC Heavyweight Champion, Frank Mir first claimed the title back in 2004, but less than two months later the MMA star was involved in a serious motorcycle accident. He suffered two breaks to his femur, and tore all the ligaments in his knee, raising doubts over whether he would ever return to UFC. Finally though, after a 17 month absence, in which time UFC had stripped him over his titles, Mir returned to the ring. He lost his comeback fight, but won back the UFC Heavyweight Championship in December 2008.

11 11. Garrison Hearst

Retired NFL star Garrison Hearst made a flying start to life in pro football with with the San Francisco 49ers in his first season in 1997. In 1999 though, disaster struck. The talented running back was spun, all of him except his left foot, which was caught in the turf. What first looked like a routine ankle break quickly showed itself to be something much more serious. Hearst had necrosis, and the bone in his ankle was dying. Surgeons quickly set about digging out the bone and replacing it with bone and cartilage from the athletes knee. Although the surgery was a success, few expected Hearst to come back and be competitive in the NFL, but after 2 years away he returned to become an integral part of the 49ers team and even transformed their form in his returning season.

10 10. Magic Johnson

The revelation that Magic Johnson had tested HIV positive shook the U.S. in a way sporting matters rarely do. The Lakers star and five-time NBA champion immediately retired upon discovering his illness. However, he didn't stop training. In fact, Magic Johnson fought back, becoming fitter and stronger in many ways than ever before, raising his bench press drastically in this time from 135 pounds to 300 pounds. His first retirement came on November 1991, but he returned for the 1992 NBA All-Star Game, where he was crowned MVP, and was also part of the U.S. 'Dream Team' at the 1992 Olympic Games. Johnson stated his intention to return to basketball on numerous occasions, often met with opposition by current NBA players, but eventually returned for the 1995-96 season, in which he averaged 15 points, 7 assists and 6 rebounds.

9 9. Ben Hogan

Considered by some to have been the greatest golfer of all time, Ben Hogan didn't always have it easy over the course of his 41 years as a pro golfer. His career was first interrupted by the Second World War, and Hogan himself became a utility pilot with the rank of lieutenant between 1943 and 1945. It was in 1949 though that was most serious blow to Hogan's career came, when he and his wife suffered a head-on collision with a bus whilst driving over a bridge. Hogan unselfishly threw himself in front of his wife, an act that actually saved his life, as the steering column punctured the drivers seat.

His injuries included a double-fracture of the pelvis, a fractured collar bone, a left ankle fracture, a chipped rib, and near-fatal blood clots. His doctors thought it unlikely that Hogan would ever walk again, let alone make a return to golf, and many of the injuries he had incurred would plague him for the rest of his life. Astonishingly, only 9 months later, Hogan made a return to golf, and went on to record 64 wins and 9 major wins, making him the fourth most successful golfer in history.

8 8. Tommy John

When you have an entire medical procedure named after you because you are the first person to undergo such revolutionary surgery, there is unsurprisingly quite a high level of risk and doubt involved. Tommy John Surgery, or 'TJS' as it is more often now known, is a procedure which involves replacing the UCL in the medial elbow with tendon from another part of the body. The surgery now has around a 90% success rate, but back when Tommy John went under the knife, doctors put his chances at 1 in 100. Having made his league debut in 1963, John had the best record of any pitcher in the league at the time of his injury in 1974, but remarkably returned and went on to play until 1989, recording more than half of his career wins after the surgery.

7 7. Niki Lauda

Not only did Niki Lauda shock people by recovering from his horrific accident at the 1976 German Grand Prix in Nuremberg, he also dumbfounded most by showing the incredible bravery and resolve to get straight back in an F1 car. Lauda had wanted to boycott the race for safety reasons, but it went ahead, and Lauda's Ferrari crashed into the embankment. Lauda was strapped into the car and could not escape the flames, leading to severe burns and the inhalation of toxic gasses, damaging both his lungs and his blood. He suffered serious scars and lost most of his right ear, but returned to F1 just six weeks later, bandaged, blood soaked and petrified, Lauda had missed only 2 races. He went on to win two F1 championships after his injury, taking his total tally to 3.

6 6. Jon Lester

Chicago Cubs pitcher Jon Lester was diagnosed with anaplastic large cell lymphoma in September 2007. It was of course devastating news for the then Boston Red Sox picture, who underwent chemotherapy throughout the off-season. The chemo was a success and delightfully, CT scans showed no signs of the disease by December 2007. He returned to training and was able to rejoin his team midway through the 2007 season, before going on to win the World Series with the Red Sox in 2007 to mark a remarkable turn of events and a perfect comeback to the sport. Lester added a second World Series in 2013 and is a three-time All-Star member.

5 5. Bethany Hamilton

Bethany Hamilton is another entrant on this list who has had her incredible journey from rock bottom to sporting achievement into a film. The young surfer from Hawaii was just 13 years of age when she had her arm severed just below the shoulder by a 14-foot tiger shark. The teenage Hamilton lost her left arm and 60% of her blood in the accident, sending her into hypovolemic shock. She returned to the water just 3 weeks after losing her arm, and began surfing once more. In 2005, a year after the attack, she won her first national title and turned pro. She has since won a further three surfing titles.

4 4. Monica Seles

Monica Seles was a tennis sensation, becoming the youngest ever winner of the French Open at 16 and winning a total of eight Grand Slam titles before the age of 20. With the most Grand Slam wins of any teenager in the Open Era, the tennis super star was on course to become one of, if not the, greatest female tennis player of all time in terms of Grand Slam wins. In 1993 though, Seles career came crashing down when a deranged fan of Steffi Graf ran onto the court and stabbed Seles with a 9 inch boning knife between her shoulder blades. Seles didn't play tennis again for two years, and although she won the Australian Open in 1996, she was sadly never able to return to her consistent and scintillating best.

3 3. Greg LeMond

The first non-European Tour de France winner and still the only official winner from the US (Lance Armstrong would have been a strong candidate for this list had he not achieved what he did in the way we now know he did), Greg LeMond is widely considered to be one of the top 10 finest road cyclists of all time. The Californian was between 1981 and 1994, but his career, and life, almost fell apart in 1987. Back home recovering from a fractured wrist, LeMond went turkey shooting, where he was shot and hit on his back and right side by roughly 60 pellets.

An air ambulance picked LeMond up, saving his life, as the cyclist had been within 20 minutes of death according to doctors. He had suffered suffered a pneumothorax to his right lung, and the bleeding was so severe he had lost 65% of his blood. His recovery surgery was a success, but few expected LeMond to return to elite cycling little over a year after the accident only to win back to back Tour de France titles in 1989 and 1990, his second and third titles respectively.

2 2. Mario Lemieux

What makes Mario Lemieux stand out above the vast majority of the athletes that have made surprising comebacks is the sheer number of hurdles the hockey star had to come. Lemieux was touted for greatness from a young age, and never failed to live up to the hype. The only person who would challenge Wayne Gretzky were it not for his injuries, Lemieux had a record of 1.883 points per game, despite regularly playing through immense pain and serious injuries, retiring with a PPG record bettered only by Gretzky himself.

Lemieux's first injury was a herniated disc, followed by rare infections. After overcoming these unfortunate hurdles it was clear Lemieux was not going to have things easy in life. He was diagnosed with cancer, Hodgkin's lymphoma to be specific, and began undergoing radiation treatment. Recovery and health was now the focus of Lemieux's life, only able to fit in hockey when his regimented recovery process allowed it, but still performing exceptionally despite the circumstances. He retirement came in 1997, when he was immediately introduced to the Hockey Hall of Fame. Remarkably, he would return again in the 2000-01 season and played another few years before retiring for good in 2006.

1 1. Rocky Bleier

Rocky Bleier's journey from promising rookie to near-fatal accident, recovery and to becoming a four-time Super Bowl champion is an epic worthy of the stage. If it were a work of fiction, one might be inclined to criticise the author of being a little fantastical and unrealistic. The Wisconsin-born halfback spent his entire career with the Pittsburgh Steelers, a love affair which began in 1968 and ended in 1980. In 1968, Bleier was drafted first by the Steelers, and then by the U.S. Army, assigned as a squad grenadier, operating a grenade launcher in South Vietnam.

The 22-year-old's service did not pass without incident, and after 3 months in South Vietnam he was shot in the left thigh by an enemy rifle. Lying wounded, a grenade then landed in close proximity. It exploded, sending shrapnel flying into Bleier's right leg, the doctors later had to remove part of his right foot. Medics informed him he would never play football again, but apparently Bleier wasn't listening. He rejoined the Steelers three years later in 1971, and a further three years later, in 1974, he finally broke into the Steelers starting line up. The resolute NFL great went on to become a four-time Super Bowl champion.