Best buddies? – Tiger Woods, right, congratulates Rory McIlroy after Europe's remarkable victory over the U.S. on the final day of the 2012 Ryder Cup in September. The two are big rivals on the golf course, but a friendship has also blossomed this year.
Too close? – Golf great Nick Faldo has questioned whether they should be so close. Sports psychologist Dan Abrahams says they would benefit from keeping a bit of distance. "In the heat of battle it becomes more difficult to emotionally detach yourself from that person's performance if you're good friends with that person," he told CNN.
Beating your idol – McIlroy has admitted idolizing Woods as a boy, but has now usurped him as golf's No. 1. "Once they step on the first tee, those competitive juices are flowing and they're focused either on their own game or beating each other," Abrahams said.
Nicklaus vs. Palmer – They are following in golf's great tradition of rivalries -- most notably Jack Nicklaus, left, and Arnold Palmer. "The power of the mind and the capability of that mental discipline is what separates the good from the great," sports leadership expert Khoi Tu told CNN. "That might allow them to become friends with people off the course, but not on the course."
Prost vs. Senna – Formula One's greatest rivalry was between Alain Prost and Ayrton Senna. "They absolutely detested each other," Tu says. "They were two very different personalities, and often that's one of the features of great sporting rivalries: Fire and ice."
Alonso vs. Vettel – Fernando Alonso, left, has now twice been beaten to the F1 championship by Sebastian Vettel -- who is widely rumored to be his teammate at Ferrari come 2014. "When you get two No. 1 drivers together with no team rules, then the sparks can really fly," Tu says. "It's rare for them to be good mates. They may get along, they may trust and respect each other in a professional capacity, but hanging out is a different issue."
F1 friendly – Former world champion Jackie Stewart, seen here playing with the children of F1 rival Graham Hill, was remarkably friendly with his peers, Tu says. "He was incredibly fond of a number of people who drove with him. That partly is because there was a clearer distinction of him being a number one and them being number two drivers."
Murray vs. Djokovic – A new rivalry has grown in tennis this year between U.S. Open and Olympic champion Andy Murray, left, and childhood friend Novak Djokovic, the world No. 1. "People say that Murray and Djokovic are close but I think it's rare," Tu says. "The best sporting rivalries are the ones where there are these very distinct, almost opposite personalities, but they're very close in terms of their competence."
Nadal vs. Federer – Rafael Nadal, left, ended the tennis dominance of Roger Federer but they have publicly expressed their friendship despite reports of arguments about on-tour issues. "As people get older they've done so much, broken lots of records, I think that competitive edge is slightly dulled," Tu says. "That makes it easier to be friendlier. You can keep your dignity if you're not crying every time you lose to a younger, faster athlete."
Navratilova vs. Evert – Martina Navratilova, left, and Chris Evert had one of the biggest rivalries in women's tennis. "They ended up as good buddies, but that was when the battling had been done," Tu says. "All of the respect and friendship you get in sport comes through competence first -- ' I know how hard it is to be this good, so I respect them for that.' "
McEnroe vs. Borg – Fiery American John McEnroe, left, and ice-cool Swede Bjorn Borg created an iconic tennis rivalry. "When those two collided, at the height of tennis perfection, that's when the audience is really enthused and enthralled," Tu says. "To get that rivalry, they have to leave any friendship on the sidelines."
Serena vs. Venus – Sometimes you get two great athletes, but they are too close to be proper rivals -- such as tennis star Serena Williams, left, and her sister Venus. "The great things about sport is the sense of competition, the uncertainty of the outcome, the fairness of the playing field," Tu says. "You might be able to suspend enough of your fraternal or sisterly love to play a decent game of tennis but it won't reach the heights of the rivalries that make the sport."
Ali vs. Frazier – Muhammad Ali, left, "demonized" Joe Frazier to hype up their fights despite being showed respect outside the ring by his rival. "He came up with the gorilla term to create a spectacle, which he knew was important, but it revved himself up too," Tu says. "He needed to have a real enemy. By the end of the (Manila) fight, he said that Frazier brought out the best of him."
Messi vs. Ronaldo – Then there's modern football's greatest rivalry -- Lionel Messi and Cristiano Ronaldo. "I'm not sure Messi is a rival with anyone, he still has that unique joy of just playing," Tu says. "But I think with Ronaldo, the truth is Messi is his nemesis -- and the fact that Messi doesn't care makes it even worse."