By Martyn Herman
PARIS (Reuters) - Roger Federer has never solved the riddle of playing Rafael Nadal at the French Open but still believes he can find the answer when the pair clash for the 39th time in Friday's semi-final.
The Swiss has suffered five French Open defeats by the marauding Mallorcan, four in finals and once in the 2005 semi-final when Nadal burst through as teenager to win the first of his 11 titles on the Parisian clay.
The last time Federer beat Nadal anywhere on clay was in Madrid in 2009 -- one of only two times he has got the better of him on the Spaniard's favourite surface.
"I knew that when I signed up for the clay that hopefully that's gonna happen (play against Nadal)," the 37-year-old Federer told reporters after his 7-6(4) 4-6 7-6(5) 6-4 victory over fellow Swiss Stan Wawrinka on Tuesday.
"Like against any player, there is always a chance. Otherwise nobody will be in the stadium to watch because everybody already knows the result in advance.
"You just never know. He might have a problem. He might be
sick. You never know. You might be playing great or for some reason he's struggling.
"That's why you need to put yourself in that position. Now I have the match with Rafa, and I'm clearly excited. I hope I can recover well in the next couple days, which I'm sure I will, and I'll give it my best shot on Friday."
Federer has played superbly on his first appearance at the French Open since 2015 and crucially he has avoided having any marathon battles on route to face Nadal.
The 2009 champion breezed through his first four rounds without dropping a set before stepping up a level to fend off Wawrinka in a high-quality contest.
He will need another level, or two, against left-hander Nadal whose vicious topspin game has given him mastery over Federer at Roland Garros where he has never even been taken to five sets by the Swiss maestro.
He is relishing the challenge though, having not played Nadal at the French Open since 2011 when he lost in four sets.
"I'm looking forward to the test," he said. "You have to be fearless to some extent to take on the spinny balls, the sliding balls, the kicking balls, and that's what I will do on Friday."
(Reporting by Martyn Herman, editing by Pritha Sarkar)