Federer is desperate to win the French Open as it is the main barrier between him and a Grand Slam.
And Nadal's third win in as many years at Roland Garros on Sunday, will make him wonder if it will ever happen.
Rafael Nadal has the upper hand against Roger Federer
The problem for Federer is that Nadal's game "matches up" pretty well against his - on all surfaces, but especially on clay.
When trying to predict results in professional tennis, you can often draw clues from how one player's game "matches up" against another.
All players have strengths; these strengths will be particular shots and patterns of play. All players (even Federer) also have weaknesses.
A player matches up well against another when their strengths and favourite patterns of play directly attack their opponents' weaknesses.
And Nadal's third win in as many years at Roland Garros on Sunday, will make him wonder if it will ever happen.
Rafael Nadal has the upper hand against Roger Federer
The problem for Federer is that Nadal's game "matches up" pretty well against his - on all surfaces, but especially on clay.
When trying to predict results in professional tennis, you can often draw clues from how one player's game "matches up" against another.
All players have strengths; these strengths will be particular shots and patterns of play. All players (even Federer) also have weaknesses.
A player matches up well against another when their strengths and favourite patterns of play directly attack their opponents' weaknesses.
In this case, the key for Nadal and the headache for Federer is Nadal's incredibly powerful and viciously spinning cross-court forehand.
The topspin on the ball makes the ball kick up high, and because Nadal is left-handed the ball is kicking up high onto Federer's backhand.
Federer's backhand is a world-class shot, but because of the grip and the technique that he uses, he prefers the ball to be a little lower.
When the ball really kicks high he can struggle - he doesn't miss, but he will often drop the ball a little short and into the centre of the court.
At this point Nadal is in charge. He moves around the ball to take it on his forehand and up the court to put on more pressure.
He now has the whole court open to him; he can hit back to the backhand side, into the open space on Federer's forehand, or he can throw in a drop shot.
When your opponent does not know where you are going to hit the ball, a clean winner will often follow.
The topspin on the ball makes the ball kick up high, and because Nadal is left-handed the ball is kicking up high onto Federer's backhand.
Federer's backhand is a world-class shot, but because of the grip and the technique that he uses, he prefers the ball to be a little lower.
When the ball really kicks high he can struggle - he doesn't miss, but he will often drop the ball a little short and into the centre of the court.
At this point Nadal is in charge. He moves around the ball to take it on his forehand and up the court to put on more pressure.
He now has the whole court open to him; he can hit back to the backhand side, into the open space on Federer's forehand, or he can throw in a drop shot.
When your opponent does not know where you are going to hit the ball, a clean winner will often follow.
This pattern happens time and time again when these top two players go head-to-head. In the 2006 final of the Italian Open, Federer came up with a game plan to counteract it.
He hit his backhand up the line as much as he could to keep the ball off the Nadal forehand and to try to get Nadal to play into his forehand.
It almost worked, but the up-the-line backhand is a tough shot and he made a few mistakes.
Nadal also started to see it coming and was able to get across to it - sometimes even managing to run around his backhand and counter-attack with the big forehand.
He hit his backhand up the line as much as he could to keep the ball off the Nadal forehand and to try to get Nadal to play into his forehand.
It almost worked, but the up-the-line backhand is a tough shot and he made a few mistakes.
Nadal also started to see it coming and was able to get across to it - sometimes even managing to run around his backhand and counter-attack with the big forehand.
