Britain's Andy Murray booked his place in the Miami Masters semi-finals on Wednesday, when third seed Andy Roddick was forced to retire with a hamstring injury trailing 5-3.

Murray, the 12th seed, was leading 4-3 when the American called for a medical timeout to receive treatment in the locker-room.

Roddick returned to the court but was immediately broken by Murray. The 2004 champion walked to the net to shake hands then informed the umpire he could not continue.

"I don't know what it is, I'm going to get an MRI, I think the medical term is, the bottom of my ass hurts," Roddick told reporters. "He hit a volley behind me and I lunged at it and hurt something.

"It's disappointing but right now I just want to focus on getting better.

"We have a big weekend next weekend and I'm kind of concerned about that."

Roddick's participation in the US Davis Cup tie against Spain next weekend has been put in doubt by the injury.

Roddick said he still hoped to lead the US in Winston-Salem but would have to wait to see the full extent of his injury.

With the victory 19-year-old Murray moves into the semi-finals for the second time in as many Masters Series events, having reached the final four in Indian Wells last week.

Murray awaits the winner of the match between second-seeded Spaniard Rafael Nadal and Novak Djokovic of Serbia.

"I guess anybody would rather get off the court quicker than longer but I'd rather it was under normal circumstances," said Murray. "I'm definitely going to have enough time off to be 100 percent fit going into the match.

"I've lost my last two Masters semi-finals and I'd like to get to my first final."