The oft-used phrase "the hardest part is letting go" could not be more perfectly applicable to the idea of an athlete retiring from a professional sports career.

When an athlete retires, they aren't simply retiring from the "x" amount of years they played in the pros. They are letting go of the entirety of the first chapter of their life - their upbringing as a star athlete, the countless hours of training and preparation, the physical and emotional sacrifices that shaped them as individuals.

The other issue that athletes are forced to face with when retiring is the fact that most are still physically capable of performing at the highest level; mentally, though, they are ready for a break. The body is ready for another summer of running, lifting and getting ready for an upcoming season, but the mind wanders to exotic beach locations, a park where the athlete can spend some long-overdue family time with his or her children, or maybe it wanders to a bedroom without an alarm clock waking them before the sun is even up, reminding them of the onset of another grueling day of pushing themselves to their limits.

They say retiring is the hardest thing an athlete will ever have to do - and judging by reactions of past athletes facing the media frenzy one last time to make the announcement, it probably is harder that anything they ever had to do to make it to the peak of their sport.

One thing is for certain though - many of them are still able to compete at the highest level, so just for fun, wouldn't it be interesting to see them come back and show these young, cocky stars how the legends used to do it, old-school?

20 20. Dontrelle Willis

Dontrelle Willis had one of the strangest baseball careers of anyone who took to a Major League baseball mound over the past 15 years. Willis was, at times during his much-too-short career, one of the most dominant pitchers in the league - other times, though, he was as wild as they would come. An anxiety disorder effectively derailed his career, but he retired at the relatively young age of 30 - now 32, Willis is in theory still young enough to get the "D-Train" back on track.

19 19. Oliver Kahn

Oliver Kahn was widely regarded as one of the best goalkeepers of his era - the list of accolades is long and impressive, featuring multiple Bundesliga championships, UEFA Best Goalkeeper awards and many other recognitions and rewards for his sparkling goalkeeping ability. Kahn has been retired for several years now, and at age 45 it might be pushing it to say he could venture back out out to the 18-yard box and still succeed - but if there was one member of the old guard we'd like back in the net, it's Kahn.

18 18. Joe Girardi

It's hard to picture Joe Girardi as anything other than a manager these days, as the only shots we get of the Yankees manager are those of him standing in the New York dugout. Ever wonder how that uniform seemed to work so well for him, while other managers have for the most part looked ridiculous wearing them? Girardi was a big league catcher for 15 seasons, so if the Yankees are ever in need of an emergency replacement, they might not be opposed to asking their 50-year old manager to strap on the shin guards.

17 17. Alex Kovalev

Alex Kovalev was probably a bit delusional in thinking that he would play (and contribute offensively) until he was 50 years old, but there's nothing wrong with dreaming big. Some might sarcastically argue that Kovy dogged it enough during his long NHL career that he has enough left in the tank to have another go at the pro game. One thing is for certain, though - the hands never go. Kovalev might not be able to keep up in today's game, but if they can't stop your dangles, they can't stop you.

16 16. Shaquille O'Neal

Shaq finds his way onto this list for a couple of reasons - one, he's still massive. His size was his biggest asset while he was dominating the post during the NBA playing career - unless he mysteriously begins to shrink overnight, then Shaq will still have that advantage against a good majority of today's NBA players. He's had a couple of years to heal and rest while analyzing games for TNT - maybe he's rejuvenated enough to get back out on the court?

15 15. Chipper Jones

Chipper Jones had a long, successful Major League career, so when he finally decided that 2012 would be his last year in the big leagues, one assumed he might take it easy and simply ride out the final year, enjoying his farewell tour and hoping to avoid any late-career injuries that might hinder the enjoyment of his retirement. Jones ended up having a pretty decent final season, statistically speaking, and even managed to attract some interest from the Yankees (among other clubs) before and during the 2013 season. Sounds like Jones is enjoying the good life, but it's clear we weren't the only ones who wondered if he could still contribute.

14 14. Chris Chelios

Chris Chelios (along with Gordie Howe) is the NHL's definition of a true ironman. Chelios, known throughout the league as a fitness freak, parlayed those offseason workouts into one of the longest NHL careers of all-time. Now 52, lacing up for an NHL game might seem out of the realm of possibility, but if Chelios has committed himself to staying in shape the way he did during his career then there's no doubt he could at least add a couple more games to the 1,651 he's already played.

13 13. Zinedine Zidane

The lasting image of "Zizou's" brilliant career was not his hoisting of the World Cup and Golden Ball at the 2006 World Cup (although he did win the Golden Ball) - rather, it was the solemn view of Zidane walking off the pitch after being ejected for headbutting Italy's Marco Materazzi in the chest in the later stages of the tournament's final. France went on to lose that game and Zidane ultimately retired, never to play professionally again. While Zidane will always be a legend in France, if there's one athlete could use a "grand finale" redo, it's Zizou.

12 12. Brett Favre

It was never a sure thing that Brett Favre would be on this list. First he was on it, but then we said "nah, forget it". He then found his way back on the list. We took him off the list again but ultimately decided that Favre should be on the list.

In all seriousness, for all the waffling Brett Favre did at the end of his illustrious playing career, we sure do miss having him around. With the upswing in beards around the NFL, Favre would fit right in among this crop of pro passers. Hopefully it doesn't take him too long to decide, though.

11 11. Paolo Maldini

Paolo Maldini spent all 25 years of his glorious football career with A.C. Milan, and any Milan fan around the world will tell you without hesitation that they could easily go for 25 more. Considered one of the greatest defenders in the history of the game, Maldini was one of the anchors of Italy's historically staunch defensive-minded style of play. Maldini racked up countless awards and trophies, but perhaps his greatest feat would be to come out of retirement (he's only 46!) and show a new generation of footballers how defending is really done.

10 10. Yao Ming

Yao Ming is one of the few (or maybe one of the many, depending on how you look at it) who managed to earn the label of one of the greatest of all-time at their position while also being remembered as someone whose career was tragically cut short by injury. Generally, it's one or the other - Ming, though, managed to put up stellar numbers in China and in the NBA. Disregarding his foot issues, if Ming were too come out of retirement there is no doubt there would be a lineup for his services.

9 9. Tracy McGrady

Tracy McGrady has earned himself the reputation as the NBA's most mysterious man over the past decade. At times, McGrady is lurking in the shadows, waiting for his next professional basketball opportunity. Other times he is playing in China, where he is revered; and he even spent some time playing professional baseball in the Atlantic League last spring. McGrady spent some time working out with Kobe this past summer, so while he might not be on the cusp of a return, there's no doubt the former All-Star still thinks he can bring it - as do we (even if it's on a very limited basis).

8 8. Ken Griffey Jr.

If Jason Giambi can play 26 games at 43 years old, you'd be hard-pressed to argue that "The Kid" couldn't recapture his youthful exuberance - and talent - at the age of 45. Griffey's time in Seattle didn't necessarily end on a high note thanks to "Napgate", but overall his talent was superseded by few others, and it's hard to believe he doesn't have a few more of those big swings left in him should he choose to lace up the cleats again.

7 7. Joe Sakic

Joe Sakic was one of those guys you simply could not hate, no matter who you cheered for. "Burnaby Joe" was a force during his time with the Quebec Nordiques and Colorado Avalanche, leading the team to two Stanley Cup Championships. Despite having been retired for the past five years, it's probably crossed Sakic's mind to lace up the skates again and show some of the young stars on his team (he's the VP of Hockey Operations in Colorado) how it's done.

6 6. Michael Jordan

Michael Jordan has done it all during his incredible athletic career - across sports and across basketball in particular. He has nothing left to prove to anyone; no one except himself, that is. Even though Jalen Rose's prediction that MJ would come out of retirement for his 50th birthday was wrong, it was still fun to think about - and we all know Jordan would have found a way to not only help his team win, but also do something spectacular, to remind everyone that he's still the king. Heck, he couldn't make his Charlotte Hornets much worse, right?

5 5. Kurt Warner

Kurt Warner played until he was 38 years old - while the age of players continues to rise across all sports, it's still a pretty impressive feat for a guy with a grey grizzled beard to be playing at a high level in the NFL. With the Arizona Cardinals barely squeaking into the playoffs after losing Carson Palmer and Drew Stanton (leaving head coach Bruce Arians to choose between Ryan Lindley and Logan Thomas), wouldn't it be something if Warner came out of retirement for another epicly unexpected Arizona playoff run?

4 4. Nicklas Lidstrom

Without a doubt the greatest defenceman of the last two decades, Nicklas Lidstrom was the definition of a pillar on the Red Wings blue-line for 20 seasons. Even in his final few seasons he found ways to remain productive and spur the Red Wings towards playoff berth after playoff berth. While the Wings remain a fairly dominant team in the East, they wouldn't be opposed to seeing no.5 back on the ice again patrolling not only the Wings zone, but the opposition's as well.

3 3. Ruud van Nistelrooy

At the still-young age of 38, there's little doubt in our minds that if Ruud Van Nistelrooy suddenly decided that he couldn't bear watching from the sidelines as a manager anymore, he could easily change the track suit for a pair of shin guards and cleats and trot onto the pitch as if nothing had changed. One of the purest goalscorers of his era, Van Nistelrooy returning to the pitch with the sole purpose of putting the ball in the back of the net (perhaps without having to move around as much) is not too far out of the realm of possibility.

2 2. Randy Moss

For starters, Randy Moss is only 37 years of age. Not only is Moss one of the greatest receivers the game of football has ever seen, he's also one of the most freakishly talented athletes the United States has produced over the last 30 years (watch the recent documentary on Moss, entitled "Rand University", and you'll understand). One does not simply lose that athletic ability - and while it may be diminished, it would still be enough to get Moss through at least a couple of big games with a half-decent quarterback.

1 1. Ray Lewis

Ray Lewis has had a pretty sweet ride through life over the past couple of years. He ended his incredible NFL career with an improbable Super Bowl Championship in 2012, and since then has been working one of the cushiest jobs in professional sport - NFL analyst for ESPN. Perhaps with a few years of rest, relaxation and rehab out of the way he can marvel us all again with his ability and his famous pre-game dance that only Lewis could make look threatening.