Professional sports are not what they were decades ago in that athletes who are rivals on the field often get along when they are not in the heat of battle. Athletes who compete in the National Football League, National Basketball Association, Major League Soccer and other organizations stand together in labor unions. They look out for one another as it pertains to business and financial matters. Gone are the days of athletes worrying about competitors “taking food off of their tables” or requiring to have second jobs during offseasons just to get by. These men are now millionaires and they vacation together when on holidays.

With that said, these are still athletes who are motivated by strong urges to win and, in some cases, desires to embarrass members of the opposition. That competitive edge that drives many of the best athletes in the world to greatness, something that we mere mortals in the public cannot fully understand, leads to real rivalries that continue long after the final whistle blows and the final horn sounds during contests. There have, throughout history, been numerous cases of wealthy individuals who have found fame and fortune in pro sports coming to blows with each other because of the hatred that burned inside of them.

It should come as no surprise to anybody who follows those sports that people who are paid handsomely to fight for a living have such hate for potential opponents. This includes what is swiftly becoming a famous feud that has crossed the invisible line that separates boxing and mixed martial arts. Men who were teammates for a long time also make the list because of the feelings of anymore that they have had for each other even though they celebrated winning championships together. Sometimes, one cannot help but to keep enemies closer than he keeps his friends.

20 20. IK Enemkpali and Geno Smith

IK Enemkpali was an unknown name even among many fans of the New York Jets at the start of 2015 NFL training camps. It will now be some time before football followers all around the United States forget the name even if they are not sure on how to spell or say it. Enemkpali became a history tidbit in August 2015 when he punched quarterback Geno Smith with such force that Smith's jaw was broken. Smith is set to be sidelined for roughly two months and Enemkpali was immediately released. Former Jets head coach and current leader of the Buffalo Bills Rex Ryan snatched Enemkpali up the next day, because Ryan really is the best.

19 19. Wayne Rooney and Wade Barrett

Some cynics would probably point out that World Wrestling Entertainment superstars are always “on,” and thus it was nothing more than a gimmick when Wade “Bad News” Barrett called Manchester United front man Wayne Rooney out in February 2015 after Rooney took a controversial spill during a FA Cup match versus Preston. It's still real to me, to steal a famous wrestling phrase, and those detractors should lighten up and have some fun with this one. Are Barrett and Rooney actually going to settle this feud inside of a ring? Probably not, but we can all close our eyes and imagine what could be.

18 18. LeBron James and Joakim Noah

One has to give Chicago Bulls big man Joakim Noah credit in that he has never backed down when facing or speaking about Cleveland Cavaliers superstar and NBA royalty LeBron James. Noah has hit out at James in interviews and during games, and James was less than pleased about it when the two faced off during the 2015 NBA Playoffs. James had this to say about Noah after one such instance: “I love his emotion as a competitor. I love his emotion, but I think the words that he used to me was a little bit too far. I'm a father with three kids. It got very disrespectful. I'm OK with competing against Jo. I love the competitive nature in him, but we should leave it there. The disrespectful words that he said to me were uncalled for so the best way to retaliate is by making a play.”

17 17. Ronda Rousey and Miesha Tate

This is the first of three times that you will see “Rowdy” Ronda Rousey featured in this piece. What started out as a friendly competition and maybe even a manufactured rivalry involving two of the top female mixed martial artists in the world became more and more real with each promo that one of the fighters would cut on her competitor. While the two have publicly stated over the years that they have a mutual respect for each other, Rousey famously summarized this feud when she stated that she does not “really feel bad about hitting (Tate).” Rousey is thus far 2-0 against Tate and the two will likely meet again at an Ultimate Fighting Championship show down the road.

16 16. Sean Avery and Martin Brodeur

Martin Brodeur and Sean Avery are two National Hockey League legends for two different reasons. Brodeur is one of the greatest goaltenders in the history of the sport, while Avery was, in his day, one of the top overall heels in all of professional sports. The hatred that grew between these two because of on-the-ice matters spread over to the world of entertainment when Brodeur mocked Avery's exit from Dancing in the Stars in March 2014. Avery responded with an old favorite insult for the former New Jersey Devils star, calling Brodeur “Fatso.” Somebody needs to get these two into a cage or a ring to settle the score.

15 15. Johnny Manziel and Brian Hoyer

Cleveland Browns head coach Mike Pettine first shed light on this feud between quarterbacks during 2014 OTAs when he joked that veteran journeyman Brian Hoyer and then-rookie Johnny Manziel would not be exchanging Christmas cards. It is now, over a year later, clear just how rough things were in the quarterbacks room of the Browns during that summer and fall. Hoyer and Manziel were polar opposites in many ways, and the perception had by national analysts and also local beat reporters in the know is that these two were more rivals than teammates. You can be sure that Hoyer and Manziel would look to out-duel each other if they ever meet on the field in the future.

14 14. Clint Dempsey and Omar Gonzalez

It is maybe the worst kept secret among those who have followed the United States Men's National Team that Clint Dempsey does not always have the sunniest of personalities, so much so that you could insert one of several names in the place of Omar Gonzalez. The US defender who features for MLS dynasty LA Galaxy and Dempsey, who plays for the Seattle Sounders, got into a shoving match on the pitch roughly one month after they were teammates at the 2014 FIFA World Cup. Dempsey has made plenty of enemies over the years, and that trend probably is not ending anytime soon.

13 13. Richard Sherman and Michael Crabtree

You can love him or you can hate him, but you cannot deny the talent that is had by Seattle Seahawks defensive back and often-controversial star Richard Sherman. The feud involving Sherman and former San Francisco 49ers wide receiver Michael Crabtree have battled on the field on multiple occasions, and it was Sherman who took a massive lead in this feud after his epic rant that followed the Seahawks defeating the 49ers in the 2014 NFC Championship Game. Sherman has undeniably gotten the better of Crabtree to date, as the Seattle man has hoisted the Vince Lombardi Trophy on a Super Bowl Sunday.

12 12. Hope Solo and Brandi Chastain

United States Women's National Team goalkeeper Hope Solo has found herself involved in several controversies over the past several years. One of those featured former US World Cup hero Brandi Chastain. Chastain was serving as a commentator for NBC during the 2012 Summer Olympics when she made some reasonable criticisms about members of the US team. Solo did not take kindly to those comments and she made her way over to Twitter to hit back at Chastain. The American goalkeeper again lashed out at Chastain in a book that was released after the Olympics. Of the two, Chastain has, in the public eye, been the classier of the athletes.

11 11. CM Punk and Ryback

CM Punk had a lot to say over the Thanksgiving 2014 weekend when he sounded off on why he walked away from the WWE earlier that year. Among the “pipe bombs” that Punk unleashed, he hinted that Ryback had used steroids and that the “Big Guy” could not be trusted to be safe during matches. Ryback went on Twitter to defend himself, and, without ever using Punk's name, he stated that the Straightedge Superstar was both “fragile” and “insecure.” Punk has moved on to the UFC, while Ryback is still featured on WWE programming such as Raw and SmackDown.

10 10. Ronda Rousey and Bethe Correia

Brazilian MMA competitor Bethe Correia thought it wise in the lead-in to UFC 190 to, purposely or not, make reference to the fact that Ronda Rousey's father had committed suicide when Rousey was younger. These comments did not sit well with the UFC Bantamweight Champion and Rousey promised that she would make Correia pay when the two met in Brazil in August. That is exactly what happened, as Rousey out-slugged the striker and then knocked Correia out cold in just 34 seconds. Pro tip for future opponents: Do not make Rousey angry.

9 9. DeAngelo Hall and DeAndre Hopkins

Can HBO program Hard Knocks help grow what could be the next great NFL feud? Let's hope so. Houston Texans wide receiver DeAndre Hopkins juked DeAngelo Hall of the Washington Redskins out of his boots during a practice session that was featured on an August edition of the HBO show, something that would have been a forgettable moment if so many television cameras would not have been there to film the moment. Hall hit out at Hopkins via Twitter after the program first aired, saying:

Hopkins responded by saying:

Get your popcorn ready, everybody.

8 8. Floyd Mayweather and Manny Pacquiao

The real-life feud involving boxing icons Floyd Mayweather and Manny Pacquiao brewed for years via back-and-forth comments before the two ever stepped foot inside of the same ring in 2015. All of that trash talk was for naught in the eyes of most boxing fans who paid up to $100 to watch the fight on pay-per-view. Rather than see a “Fight of the Century” between two hated rivals, viewers were instead given a boring hug-fest that made customers beg for a Ronda Rousey run-in that ended with her tapping both guys out. It is rumored that Mayweather and Pacquiao may have one last battle to settle the rivalry. No thanks.

7 7. Jose Mesa and Omar Vizquel

Shortstop Omar Vizquel and closer Jose Mesa were close friends when playing for the Cleveland Indians in the 1990s. That all changed after Vizquel criticized Mesa for blowing the save in Game 7 of the 1997 World Series, a game that the Indians ultimately lost to the Florida Marlins, and Mesa responded by promising to plunk Vizquel whenever they faced off. Mesa proved to be a man of his word, hitting Vizquel with pitches on three occasions. Time may heal old wounds or at least that is what the saying preaches, but these two won't be sharing a holiday meal together.

6 6. Serena Williams and Sloane Stephens

One would have to look far and wide to find a more competitive professional tennis player than Serena Williams and her nature could have something to do with why this relationship fell apart over time. Williams and Sloane Stephens were close friends, at least they were until Stephens upset Williams at the 2013 Australian Open. This proved to be a turning point for the once mentor and once student of the all-time great. Stephens and Williams have both hit out at each other since that fateful day down under and all indications that the positive feelings that were once had between the two have disappeared.

5 5. Zlatan Ibrahimovic and Rafael van der Vaart

Front man Zlatan Ibrahimovic has been as eccentric and popular as he has been controversial during the bulk of his career. The man who has asked fans to “Dare to Zlatan” via social media campaigns was involved in a dust-up with Holland midfielder Rafael van der Vaart in 2004 when Ibrahimovic injured the Dutch maestro. Van der Vaart insisted that the injury was no accident in interviews and also when confronted by Ibrahimovic over the matter. The forward held his ground and offered the following threat to Van der Vaart: “I didn’t injure you on purpose, and you know that. If you accuse me again I’ll break both your legs, and that time it will be on purpose.”

4 4. Michael Jordan and Former Teammates

Michael Jordan is widely perceived to be the greatest basketball player to ever take the court. Part of what made Jordan untouchable during his prime was that he was the ultimate competitor 24/7, even when practicing with teammates. Odds are that you have heard stories about Jordan getting into fights with Will Perdue and Steve Kerr when they all were members of the Chicago Bulls. Jordan wanted to win at all costs at everything that he did, and that fire helped make him a legend of the sport. It also made Jordan an enemy among some who played alongside of him during the 1990s.

3 3. Derek Jeter and Alex Rodriguez

The hope over a decade ago among fans of the New York Yankees was that the shortstop and living legend of the club would become fast friends with the slugger. While the two did celebrate winning a World Series championship together, it is no secret that Derek Jeter and Alex Rodriguez only grew further and further apart with every season. A-Rod had less-than-flattering words for Jeter before Rodriguez linked up with the Yankees and that certainly did not help matters. Jeter has not wanted to even speak about Rodriguez since retiring as an active playes and these two former teammates are now essentially strangers off of the diamond.

2 2. Kobe Bryant and Shaquille O'Neal

It is arguably the most famous feud among former teammates in the history of the NBA. A Wikipedia page, chapters in books and articles have been dedicated to the heat that Bryant and O'Neal have had since when they were together on the roster of the Los Angeles Lakers. It has even been suggested that Shaq once threatened to murder – yes, kill him dead – Bryant during the height of the feud. Bryant, who has never met a basketball shot that he did not like, isn't the greatest of teammates. We know this. He nevertheless probably does not deserve to be taken out for good by any current or former player.

1 1. Ronda Rousey and Floyd Mayweather

The baddest woman on the planet versus the boxer with a history of domestic violence accusations. Good versus evil. Rousey and Mayweather have taken verbal jabs at each other while attending events and when presented with other opportunities to make such comments. Rousey has gone so far to say that she could defeat the boxer if the two were to meet inside of the cage and she is not the only person out there who believes that the "Rowdy" one would emerge from that encounter as the victor. The fight is never happening for the obvious reasons, but Rousey putting the armbar on Mayweather would make for maybe the greatest SportsCenter highlight ever.