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.

Friendly enemies

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.