He is truly gutted, what a sad laddie in the photos. ESPN on his FB. No personal comment.
"I was struggling to walk," said Murray, the world number three.
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. 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."