Basel - Andy Murray withdrew from the Swiss Indoors on Wednesday shortly before his match with Dutchman Robin Haase with a right buttock muscle strain.
The world No 3 was replaced in the draw by Swiss Marco Chiudinelli.
The pullout came after Murray, winner of three titles last month in Asia, had requested a wild card into the event featuring rivals Novak Djokovic and Roger Federer.
Murray said that he woke up around 3 am on Tuesday with the pain in his buttock.
"I was struggling to walk," said Murray.
"I had trained twice on Monday and felt fine after that.
"It was a bit better later on Tuesday morning and I went to a pool for some exercises and had a light hit. But this morning (Wednesday) I knew it was still not good enough.
"I don't know how I did it or what it came from. I've never had anything like this before."
Murray's withdrawal opens the door for Basel-born Chiudinelli, whose wild card was ironically withdrawn by the tour in order to give it to Murray at late notice last week.
The Scot said that his personal physio and doctor suspect the problem might be linked to the sciatic nerve.
Murray said he will travel to Paris on Thursday and take four or five days off with anti-inflammatory treatment in hopes of being fit for the final event of the regular ATP season starting on Monday, the Paris Masters.
Murray's pullout was the second of the day after Serb sixth seed Janko Tipsarevic was unable to go on when he was trailing 5-1 in the first set against German Florian Mayer in their first round match.