Eight games into the match today, Roddick had decided it was time to quit.

He retired with a strained left hamstring in the first set, allowing Murray to advance to the semifinals of the Ericsson Open.

Roddick said the injury occurred while lunging at a shot in the fifth game. He called it quits trailing 5-3.

"I just kind of jarred something at first," he said. "More and more it started becoming like a sharp pain, and then I started feeling it even when I was just kind of cruising around the court, not even in points. It was steadily getting worse, which isn't a good sign."

An MRI exam revealed a grade-one strain — the least severe kind, with grade three the worst. Still, the injury leaves in doubt Roddick's availability for the U.S. Davis Cup team's match against Spain on April 6-8.

"It's a hamstring — it could take three days, or three weeks," U.S. captain Pat McEnroe said. "We're going to do everything we can to get him ready. There's no tear, that's the main thing."

Roddick said he thought of Davis Cup the moment he hurt himself.

"It's disappointing," he said. "Right now I just want to focus on getting better."

Roddick failed to react to several of Murray's shots, took a medical timeout at 4-3 and left the court.

"It's hard to concentrate, because you don't know how bad the injury is," Murray said. "You just have to try to get the ball back and make them play."

Roddick returned to play six more points but appeared hobbled. On the final rally, he hit a half-volley at the net. When Murray pulled a backhand crosscourt for a winner, Roddick didn't move.

Roddick then conceded by shaking hands with Murray.

"Anybody would rather get off the court quicker than longer," Murray said. "But I wish it was in normal circumstances."

Roddick retired with a wrist injury in his opening match at the 2005 tournament. He won the title in 2004.

Murray became the first Brit to reach the Key Biscayne semifinals since Tim Henman in 1998. The Scotsman is ranked a career-high 12th and leads the men's tour with 23 wins — two against Roddick — in 27 matches.

Murray's opponent Friday will be the winner of the match Wednesday night between second-seeded Rafael Nadal and No. 10 Novak Djokovic.

No. 14 Shahar Peer advanced to the women's semifinals, winning the first nine games and beating No. 24 Tathiana Garbin 6-0, 6-3.

Peer's opponent Thursday will be the winner of the match Wednesday night between three-time champion Serena Williams and No. 8-seeded Nicole Vaidisova. Peer nearly pulled off an upset against Williams in the quarterfinals of the Australian Open in January, losing the third set 8-6.

Peer, a 19-year-old corporal in the Israeli Army, has a 20-6 record this year and is ranked 16th. She won 31 of 55 points on Garbin's serve and broke five times.