"It comes down to executing your game plan and the conditions," Lendl told reporters on Thursday.
"Andy has been putting in great work. If you have three illnesses and an injury which take you out for some time, it's very difficult mentally for the player...
"But now he is healthy and he can put the practices in."
Murray, who has a 21-7 record this year, said his indifferent form earlier in the season had made him question things but he had been able to just focus on playing in recent matches, which had made a huge difference.
"I'm thinking less on the court. I think when things aren't going well, it's very easy to overthink things. You can be worrying about technique, which is never good when you're playing matches," Murray said.
"I was still feeling a little bit like that at the beginning of the tournament, the first match (against Andrey Kuznetsov). But I got through that and it made a huge, huge difference to me."
Murray, who was runner-up to Novak Djokovic at Roland Garros last year, faces 2015 champion Wawrinka later on Friday.
(Reporting by Aditi Prakash in Bengaluru; Editing by Peter Rutherford)
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