British number one Andy Murray has pulled out of the Swiss Indoors in Basle because of a right gluteal muscle strain.
The 24-year-old, who is on a 15-match winning streak, withdrew shortly before his first-round match against Dutchman Robin Haase was due to start.
"I was struggling to walk," said Murray, the world number three.
Roger Federer made progress to the quarter-finals with a 6-1 4-6 6-3 win over Finland's Jarkko Nieminen.
Murray, who won three titles in Asia last month, had requested a wildcard to play in an event featuring world number one Novak Djokovic and defending champion Federer.
"I trained twice on Monday and felt fine after that," he said. "It was a bit better 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 said he will travel to Paris on Thursday and take four or five days off with anti-inflammatory treatment in the hope of being fit for the Paris Masters, the final event of the regular ATP season, which starts on Monday.
The Scot was replaced in the draw by Swiss Marco Chiudinelli, who subsequently lost the match against Haase 6-2 7-6.
Murray had chosen to play in Basle over Valencia, where he was champion two years ago, and was also set to play in the doubles with his brother Jamie.
He had been looking to add to the titles he recently won in Bangkok, Tokyo and Shanghai, which enabled him to climb above Federer in the rankings.
Murray, who also won the Cincinnati Masters title in August, has only suffered one defeat in his last 26 matches - that came against Rafael Nadal in the semi-finals of the US Open last month.