Murray suffered a few lapses in concentration and knows he must do better against Rafael Nadal.
The fourth-seeded Briton, who twisted his ankle in his third-round match with 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.
Murray, who has never won a grand slam title, was more consistent against unseeded Argentine Chela in the third set and sealed the win with a delicate drop shot.
"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 towards 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 favour.
"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, looked on the brink of retirement when he twisted his ankle in a five-set victory over Serb Troicki.
"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.