THEY are the formidable four of men's tennis.

Between them Rafael Nadal, Novak Djokovic, Roger Federer and Andy Murray have shared out 37 of the last 40 grand slam titles but the one opponent none of them can outrun is time.

As the topic of conversation turned, as it so often does, to that mythical changing of the guard moment, that inevitable instant where one generation overtakes the other, perhaps Murray summed it up best. "It is only a matter of time really," he said, before the battering at the door of Grigor Dimitrov, Milos Raonic and others reaches a crescendo. "But how long it is going to take? You just don't know."

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What we do know is that tennis has become a gruelling endurance sport where veterans can be rendered obsolete in their mid 30s. Federer, who will turn 34 in August, is a genetic freak, chasing one last shot at glory while two sets of twins claw away at his practice time.

Nadal blew out the candles on his 28th birthday cake earlier this month, dreaming of further eating into his Swiss rival's three-slam lead (17 to 14) at the top of the Greatest Of All Time stakes. Djokovic and Murray, born a week apart in 1987, have been forced to find ever more creative ways to balance their pursuit of further glories with the rigours of training on their bodies.

So who are these likely lads, and just how likely is it that their big breakthrough will occur this fortnight?

For John McEnroe at least, the two men at the front of the queue are Dimitrov and Raonic. At 23, they are hardly in the rookie category, indeed both may feel their moment is already overdue. The Bulgarian's grass court prowess was confirmed by his recent capture of the Queen's Club title while the Canadian who left Montenegro as a child has the kind of big-serving game which could force the bigger guns into many tie-breaks.

With the 2009 US Open winner Juan Martin del Potro absent with an injured left wrist, such an analysis, of course, slights the Australian Open winner Stanislas Wawrinka - the only man in the field outwith the big four who has a grand slam title to his name - although he has been struggling for fitness and is largely overlooked as he possesses a game which is not suited to grass.

"The four top guys are still the obviously choices," said McEnroe, now a pundit for broadcasters ESPN and others. "But Dimitrov and Raonic aren't long shots anymore. These are the two guys that I think have made the biggest advances, who we've been waiting for the longest to potentially do some serious damage at a major event. They're starting to show that if they get a little bit of luck, if things fall their way, then they could make a real run in this tournament."

If Amelie Mauresmo's arrival can help provide an upturn in Murray's chances, McEnroe feels the influence of girlfriend Maria Sharapova can already be seen in Dimitrov. The agency of highly regarded Australian coach Roger Rasheed has also helped the player to three tour titles this year.

"I like a lot about him," said McEnroe. "What I didn't like about him was it didn't seem he was dedicated enough compared to what the other top guys were doing. But being around Sharapova I bet has helped him, because he's happier and because she's so dedicated.

"I call her the Nadal of the women's tour because she plays every point like it's her last point and that has to have rubbed off.

"Also Rasheed is known as a fitness guy so Dimitrov has realised over the course of time if he wants to make a mark, he has to be fit.

"You can't expect to be at the end of majors or winning them if you can't last till the end of a best-of-a-five-set match.

"He hasn't exactly knocked them dead in the majors but it looks like he can at least go the distance now."

Nadal and Federer, of course, fell in the first couple of rounds 12 months ago and Djokovic is another who feels this year could be even more unpredictable.

"I think it is going to be an open tournament," the Serb said. "I feel like there is a new wave of players, especially the younger generation, like Dimitrov and Raonic and [Kei] Nishikori, [Bernard] Tomic, these kind of players who have proven before that they can win against the top guys in the big events. This could easily happen here."

Indeed it could, but don't bet on it just yet. Any outside challenger would have to beat the 'fab four' and the odds. The bookmakers will give you 16/1 against any new name being engraved on that famous old trophy in a fortnight's time.