John McEnroe believes Novak Djokovic is feeling the pressure of trying to become the first man for 43 years to hold all four grand slam titles.
Not since Rod Laver in 1969 have the trophies been in the possession of one man, and Djokovic will face his latest hurdle on Friday when he meets Roger Federer in the semi-finals.
Neither man has found his best form yet at Roland Garros, but Djokovic has lived the most dangerously, coming from two sets to love down to beat Andreas Seppi in the fourth round and then saving four match points against Jo-Wilfried Tsonga.
And even if the Serbian beats Federer he is then likely to meet Rafael Nadal, who faces fellow Spaniard David Ferrer in the other semi and is going for a record seventh title.
McEnroe said: "Rafa is the guy that's come in playing the best. The second best, in terms of how well he's played, is Ferrer - this is the best I've ever seen him play. So those two guys are coming in with a lot of confidence.
"As far as Novak and Roger are concerned, both guys have not really got their total A game going. Novak I think is really feeling the pressure. It's impossible not to feel that because he's so close.
"Now that he's got here, I expect him to play better. Even though he lost to Roger here last year, it's going to be Roger that's going to have to play quite a bit better in order to win.
"But both of them are going to have to play even better if they want to win the final."
Were Djokovic to win on Sunday, he would still have work to do to match Laver, who won the true Grand Slam of all four titles in a calendar year on two occasions, one of only two men to achieve the feat along with Don Budge.
McEnroe added: "It's not technically the Grand Slam but I'd come up with a statue or something that said I had four in a row and I'd be parading it around my house for the rest of my life.
"Then I'd go and try to win the third and fourth, and then there would be no question about it. But man, that would be just fine with me."
The women's final tomorrow will be between Maria Sharapova and surprise package Sara Errani, who beat US Open champion Sam Stosur 7-5 1-6 6-3 to reach her first slam final.
Sharapova, a 6-3 6-3 winner over Wimbledon champion Petra Kvitova, is looking to complete her set of slam titles and is also now guaranteed to overtake Victoria Azarenka as world number one on Monday.