By Julien Pretot

PARIS (Reuters) - Andy Murray suffered lapses in concentration in a 7-6 7-5 6-2 win over Juan Ignacio Chela on Wednesday to reach the French Open semi-finals for the first time -- and knows he must do better against Rafa Nadal.

The fourth-seeded Briton, who's campaign had looked in danger after twisting his ankle in a third-round win over Michael Berrer, recovered from two breaks down in the opening set to win a tiebreak 7-2 and blew a 5-2 lead in the second before getting his act together.

"It was just very up and down. The wind obviously doesn't help but it was a really scrappy match," the Scot told a news conference.

"I didn't start particularly well and then got a little bit better, started moving a bit better toward the end of the first set."

Murray must be switched on from the start to stand a chance of booking a place in the final because he faces world number one Nadal on Friday after the five-times champion beat Swede Robin Soderling 6-4 6-1 7-6.

"I got up in the second, maybe lost concentration a little bit, which you can't afford to do against someone like Juan who has a lot of experience on this surface. Something I definitely won't get away with against Rafa," he added.

Murray possesses a seemingly nonchalant attitude but has shown plenty of fighting spirit on a slow surface he does not favor.

"I'm surprised I'm here, to be honest, because I haven't actually played that well. Aside from everything else that's happened I haven't played particularly well," he said.


Murray, the Australian Open runner-up, had looked on the brink of retirement against Serb Viktor Troicki in the last round as he struggled with that ankle injury, but came through in five sets.

"I needed to fight back against Troicki in tough circumstances. I think all my matches except that one were straight sets," said Murray.

"I'm glad I've got tomorrow off where I can rest and recover ... because you're going to need all of your reserves to get through a match with Rafa at the French."

Nadal has a 43-1 record at Roland Garros. Murray has a 4-10 record against the world number one and has never beaten him in three encounters on clay although the Spaniard dropped a set at the Monte Carlo Masters this year when they met in the semi-finals.

(Editing by Tony Jimenez)

(This story is corrected to clarifying that Murray incurred the ankle injury in his third round match against Berre)