Rafa Nadal arrives at Roland Garros without his air of invincibility as the seemingly unstoppable Novak Djokovic threatens to flatten the Spaniard's bid to equal Bjorn Borg's six French Open titles.

The Serb's astonishing form during a 37-0 start to the year, including consecutive victories on clay for Djokovic over the world number one, means that for the first time since 2005 Nadal is not the overwhelming favourite for the claycourt slam.

Since Nadal first bounded on to the scene with muscles bulging from his cut-off shirts and pirate shorts he has, for all but one match, looked unbeatable on Parisian brickdust.

Every challenge thrown his way was answered in emphatic style as he stormed to five titles with a win loss record of 38-1, the sole defeat coming in an injury-hit 2009 against Sweden's Robin Soderling when his body betrayed him.

With Borg's record looming, however, Djokovic looks to have erected a barricade that even Nadal's formidable claycourt armoury suddenly looks ill-equipped to dismantle.

Successive victories over the Majorcan powerhouse in the finals of the Madrid and Rome Masters, having never beaten him on red dirt before, have changed the lay of the land.

In Rome, in particular, there were times when Djokovic appeared to have Nadal on the end of a piece of string, pulling him one way and another to his heart's content.

John McEnroe, the former world number one and seven-times grand slam champion, thinks Nadal must go back to the drawing board in search of a new strategy.

"I think the way he approaches matches against Djokovic he is going to have to think about changing some of the ideas he had before the last couple of matches," McEnroe, whose record 42-match win streak from the start of the year ended in the French Open final in 1984, told reporters this week.

"He has to take a few more chances, get Djokovic on the defensive a bit more, instead of letting Djokovic dictate play and banking on him missing.

"It seemed like he got pushed around a bit (in Madrid and Rome) and I'm sure his camp will go back to the drawing board.

"Nadal came off one of the greatest years in Open tennis history and now all of a sudden he finds himself befuddled and baffled at what to do when he plays against Djokovic."

When Nadal beat Djokovic to win last year's U.S. Open and complete his career grand slam it appeared inconceivable that less than nine months later he would be almost resigned to losing his world number one ranking to the surging Serb.

A ranking points gap measured in thousands has been whittled down to 400 and should Djokovic get his hands on the Coupe des Mousquetaires trophy for the first time he will find himself out on his own at the top.

"I think this is an exciting time for our sport," added McEnroe, who still cautioned against making Djokovic outright favourite for the title.

"I would say he's earned the right to be co-favourite. Nadal has won it five times and is still incredibly difficult to beat and his record at the French Open is phenomenal."

With Djokovic and Nadal hogging the limelight ahead of the French Open, 2009 champion Roger Federer, finds himself in the rare position of arriving for the start of a grand slam as an "outsider" although the Swiss will still be dangerous.

Last year the 16-times grand slam champion was overpowered by eventual runner-up Soderling in a damp quarter-final tussle and his claycourt preparations this year have included defeats by Juergen Melzer and Richard Gasquet.

"A lot of things can happen in majors," McEnroe said of world number three Federer's chances.

"The chance of him beating both Djokovic and Nadal back to back at this point of his career is not high, however if something were to happen where one or both didn't reach as far as expected then Roger has the experience to take advantage."

Australian Open runner-up Andy Murray could also come into contention while Soderling, runner-up for the past two years, and Spain's David Ferrer will fancy their chances.

Murray, the world number four, suffered a horrible slump after being thrashed by Djokovic in the Australian Open final but appears to have regained his confidence just in time and came within a game of ending Djokovic's run in the Rome semis.