- Golf stars Rory McIlroy and Tiger Woods talk to CNN about their growing friendship
- Sports leadership expert says their rivalry depends on not becoming too close
- Empathy for fellow competitors can dull their "killer instinct"
- Psychologist says some top European golfers have been "too friendly"
Rory McIlroy and Tiger Woods, Novak Djokovic and Andy Murray, Sebastian Vettel and Fernando Alonso -- this year has witnessed the evolution of some classic rivalries that bode for an intriguing 2013 sporting calendar.
Will they become the new Nicklaus-Palmer, Borg-McEnroe, Federer-Nadal or Prost-Senna -- or will an apparent tendency towards friendliness dampen the sparks that are so vital in a true clash of competitive egos?
The relationship between Woods and his heir apparent as golf's biggest star is already showing signs of an unexpected "bromance".
"I think it's sort of evolved since Abu Dhabi at the start of this year," McIlroy told CNN's Living Golf when the duo sat down for a joint interview.
"I'd played with him before but never really got a chance to speak to him in depth. I think we both have a lot in common -- we're both big sports fans and I think our relationship has evolved from there because we've played together quite a lot this year.
"It's been great for me to get to know him and maybe try and pick up a few things and learn from him too."
Before the rise of McIlroy, golf's undisputed No. 1 player this year, Woods' previous big rival was Phil Mickelson -- and their relationship was notoriously frosty.
They snapped at each other via the media, and Woods' former caddy Steve Williams ramped up the tension with some choice remarks to the press and and fans from which even his employer had to hastily distance himself.
But Woods, now 36, seems much happier in competition with McIlroy despite the 23-year-old being poised to usurp him as Nike's biggest golf endorsement.
"We've battled each other a few times, but we have a lot in common," Woods said. "Granted, there's an age difference but I had a huge age difference with my other good friend Mark O'Meara, but we had so much in common.
"I think our relationship will certainly grow over the years, but so too will our competitiveness -- I don't think that's going to change."
However, while Woods is good friends with O'Meara, a man almost 20 years his senior, they were never really on-course rivals despite the older American's two major triumphs in 1998 -- a year of highs that he never repeated.
But if Woods and McIlroy are to become truly great adversaries along the lines of Nicklaus-Tom Watson and Ben Hogan-Sam Snead then they can't afford to become too cosy, according to sports leadership expert Khoi Tu.
"At the extreme, the ability to defeat your opponent, to crush them, requires huge mental discipline," said Tu, whose book "Superteams" features the F1-dominating Ferrari/Schumacher era and the 2010 Ryder Cup-winning team captained by European golf icon Colin Montgomerie.
"As soon as you begin empathizing with your competitor, you may not have that killer instinct," Tu told CNN. "Great sporting rivalries bring out the best in both players. Great sporting friendships is a great tactic for one and not the other.
"If Rory becomes a genuine rival as opposed to the rival of the moment, then I think Tiger will find it hard not to compete in every dimension possible.
"It's in his DNA, it's what makes him a great competitor. In many respects the best thing about it would be if they did become real rivals, if there was a sense of abrasion or friction -- a sense that sparks fly. It would do wonders for the sport."
As Nicklaus once said of Palmer: "We are adversarial friends or friendly enemies. All our lives we've competed against each other. Arnold and I fight like the devil about stuff."
Tennis stars Murray and Djokovic have been friends since childhood, but they have still maintained a ferocious on-court rivalry that this year has reached towards the heights of predecessors such as Federer-Nadal, Borg-McEnroe and Navratilova-Evert.
Roger Federer at first struggled when Rafael Nadal ended his complete domination of the men's game, but the duo now publicly insist they are firm friends following years of epic clashes -- despite an apparent spat this year over the Spaniard's desire for changes to be made on the ATP Tour.
"Great shared experiences build great bonds between people," Tu says. "That bond is different than friendship. It's a bond of respect."
For Woods and McIlroy there is every incentive to cast themselves as friends and rivals.
Already they have been lured into playing a lucrative exhibition clash in one of golf's key markets, China, that led them to withdraw from a premier European Tour event in the same country later that week.
"They both just got huge paydays in China, they may be putting a slight front on that camaraderie," sports psychologist Dan Abrahams told CNN.
"In a sport like golf, having a spat with a rival isn't seen as being the right thing, whereas (in soccer) Alex Ferguson having a spat with (rival manager) Arsene Wenger, that seems to be quite normal."
Abrahams argues some of McIlroy's older European peers such as Luke Donald, Lee Westwood and Darren Clarke -- who have won only one major title between them -- may have suffered from their on-tour friendship.
"Speaking with a lot of golf coaches, that generation have been perhaps a bit too friendly with each other," said Abrahams.
"If you compare them with Nick Faldo, he was renowned for keeping himself quite isolated, which was a catalyst for his competitiveness. I think keeping your distance from competitors can be a useful thing."
Faldo has questioned the apparent friendship of golf's two biggest names, but McIlroy played down the criticism.
"You're on tour long enough and you don't need enemies out there, you want to have friends," the Northern Irishman told CNN.
"Life on tour can get a bit lonely at times and you wanna have guys that you can go out for dinner with. You're seeing most of the guys each week and you've got to have someone to talk to. If that's what worked for Faldo then great, but I don't think it would work for me."
In the high-octane world of Formula One, Alonso is known for his clashes with fellow drivers -- he left McLaren after only one season following a bitter feud with teammate Lewis Hamilton.
However, this year the Spaniard has been much more respectful of Vettel, despite Ferrari's united front in downplaying the achievements of Red Bull's history-making triple world champion.
Rumors persist that the Italian marque wants to sign Vettel for the 2014 season, which would be an acid test for relations between the two drivers.
"When you get two No. 1 drivers together with no team rules, then the sparks can really fly," says Tu, who has worked with Ferrari and former F1 world champion Jackie Stewart.
"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."
Seven-time world champion Michael Schumacher, who has now retired for a second time, was known for his win-at-all costs approach at Ferrari but such an attitude is becoming increasingly unacceptable for sports fans, says Tu.
Contrast the feelgood factor of the London 2012 Olympics with the demonization of Lance Armstrong, who has refused to contest allegations of systematic doping during his time as cycling's tour de force.
"There is, at the moment, a very fine line that sport is treading -- this desire and ambition to win, but not to do so at all costs," says Tu.
"Someone like Schumacher, his desire to win would take him to the dark side. That type of willingness to do whatever it takes is a feature of champions. Unmoderated, it's pretty dangerous for the game.
"We've just been through a period of excess and egregious behavior from many corporates -- but equally sporting teams as well. The desire to win, and the merits and rewards of winning, maybe outweighed the joy of winning.
"I think sport is recalibrating. That's why the Olympics made everyone so happy. There was a sense that winning was important, but not necessarily at all costs -- it's the sportsmanship sometimes that makes the bigger story."