Recent form will vie with historic dominance when Rafael Nadal and Roger Federer renew their rivalry in the French Open semi-finals on Friday.
Nadal has won 23 of their 38 previous matches including all five at Roland Garros and 13 of 15 on clay, but Federer has come out on top in their most recent five meetings, all on hard courts.
Having not beaten his great rival at a grand slam since Wimbledon 2007, Federer claimed a hugely important victory over five sets in the Australian Open final in 2017 to shift the dynamics of their match-up once again.
Now 37, Federer must hope the confidence from those wins and his impressive return to clay can allow him to find the answer to a riddle he has spent most of his career trying to solve.
"It's never natural against any lefty," said Federer. "It's just everything changes. We play 80 per cent of the time against (right-handed players). And when we play a lefty, it's just a different match. It's an interesting match.
"I used to hate it. Now I love it, because it's a huge challenge against those guys, and he's the best one that I ever faced. I'm looking forward to the test."
Federer survived a real examination of his clay-court game in his quarter-final against good friend Stan Wawrinka, dropping his first set of the tournament but coming through in four.
He spent a lot of time at the net, as he has all tournament, and Nadal does not expect any surprises from his old foe.
"I really expect that he's going to play aggressive, changing rhythms, going to the net," said the Spaniard. "That's my feeling, that he's going to try to play that way, because he's playing well and he has the tennis to make that happen.
"I have to be solid. I have to hit the ball enough strong to not allow him to do the things from good positions. I need to let him play from difficult positions, so from there he's going to have less chances to go to the net or to play his aggressive game.
"If I am able to play good tennis and play well with my forehand and backhand, I hope to put him in trouble. If not, I will be in trouble."
It would be one of the biggest achievements of Federer's career if he could beat Nadal and reach a first final here since 2011 after four years away from the Paris clay.
Federer has made the final here five times, losing four times to Nadal and winning his only title in 2009 with victory over the Spaniard's conqueror Robin Soderling.
The closest he came to defeat that year was in the fourth round when, following Nadal's shock defeat, he trailed his close friend Tommy Haas by two sets to love before recovering to win in five.
Haas is now competing in the veteran's event and believes his former rival is playing better than ever.
"He's very sharp, moving really well," said the German. "That's always the key for him, especially on clay. I think he's already very satisfied with coming back and deciding to play the clay-court season, he's getting a lot of matches under his belt.
"It's truly amazing what he's doing and if he's healthy and feeling really well then watch out, he can be really dangerous.
"He will go in there with nothing to lose. I think Rafa will have most of the pressure on him. If Roger comes out firing, getting a good start and possibly winning the first set, I think we'll all be glued to the TV to see the ending to that one.
"I think the main goal anyway will be to win Wimbledon and with that kind of process and matches in his belt will help him as well. We're still talking French Open, he's still alive so we'll see what happens."
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