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."