does not sound too serious...but still...
"The hard court is, for me, the hardest on my body and it always takes me a bit of time to getting used to playing on them again and I hope it doesn't affect the next few weeks."
Murray, whose triumph at the London Games was on grass, suffered the injury while chasing down a forehand and was grimacing on the court after repeatedly grabbing his knee.
He called a medical timeout while leading his match with Italy's Flavio Cipolla 6-1 3-2 and received about five minutes of treatment before returning to the court to finish.
Murray, who made the decision to withdraw after a warm-up on a bike earlier on Thursday, said his physiotherapist will be at next week's Cincinnati Masters where he will have more treatment and training before deciding how to proceed.
"The left knee, I've never really had many problems with it beforehand so that was another reason for making this decision. I want to make sure it's not something that I am going to play through and maybe damage a little bit more," said Murray.