John McEnroe gives Novak Djokovic a slight edge ahead of his French Open semi-final rematch with Roger Federer on Friday night.

Tennis great John McEnroe senses Novak Djokovic is feeling the weight of history, but still gives the Serb a slight edge over Roger Federer heading into their blockbuster French Open semi-final on Friday.

Bidding to become the first man since Rod Laver in 1969 to hold all four grand slam trophies, Djokovic had to fight off four match points in a five-set quarter-final cliffhanger against Jo-Wilfried Tsonga after Andreas Seppi also took the world No.1 the full distance in the fourth round.

"There's nothing obviously better than the chance of doing something that hasn't been done for 43 years and having four majors in a row," McEnroe said on Thursday.

"So that alone makes it almost impossible to play as well as you can because the pressure alone and the expectation to have to be able to do that and execute it is almost impossible."

Federer last year snapped Djokovic's 43-match winning streak in the Roland Garros semi-finals and is once again plotting to end the Serb's campaign at the penultimate stage.

"Novak, I think, is really feeling the pressure like he did last year. It's impossible not to feel that because he's so close," McEnroe said.

"I guess Roger was this close - he had won three in a row and was in the finals twice. I believe Rafa was in the same position in Australia (last year).

"They could speak of that better than I could. I was never holding three in a row and going for that fourth one.

"But I have been around enough big occasions, I know that's got to start to get to you a little bit.

"It did obviously when he played Seppi. It was like he really was struggling.

"Even against Jo he wasn't maybe as sharp as we've seen him. That's not unexpected.

"Now that he's gotten here, I expect him to play better."

But McEnroe - whose sole grand slam loss in his extraordinary 1984 season came from two sets up in the French Open final against Ivan Lendl - feels Federer too is battling after the Swiss master needed five sets in his quarter-final win over Juan Martin del Potro.

"Both guys have not really gotten their total A game going," he said.

"It's difficult to say what's going to happen, if one or both of these guys could and may step it up.

"Everyone expected this match. When it happens, maybe they can sort of find their best level.

"There is no reason to believe that Roger won't play better. He has to play better if he's going to win."

McEnroe said regardless of who prevailed, both Djokovic and Federer would be a big underdog to topple Rafael Nadal in Sunday's title match, presuming the six-times champion sees off fellow Spaniard David Ferrer in Friday's other semi-final.

"First of all, Rafa is the guy that's come in playing the best," the former world No.1 said.

"The second best, in terms of how well he's played, is Ferrer. It's like the best I've ever seen him play right now.

"So those two guys are coming in with a lot of confidence."