updated 4:19 p.m. ET March 29, 2007

KEY BISCAYNE, Fla. - As the shot sailed past Andy Roddick, he grimaced in pain and didn’t even try for the ball. Instead, he limped toward Andy Murray at the net and extended his hand.

Eight games into their match Wednesday, Roddick had decided it was time to quit.

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

Three-time champion Serena Williams also reached the semifinals, beating No. 8-seeded Nicole Vaidisova 6-1, 6-4.

Williams will play for the fourth day in a row Thursday night when she faces No. 14 Shahar Peer. A 19-year-old corporal in the Israeli Army, Peer nearly upset Williams in the quarterfinals of the Australian Open in January, losing the third set 8-6.

Murray’s opponent Friday will be No. 10 Novak Djokovic, who beat No. 2 Rafael Nadal 6-3, 6-4. Djokovic lost to Nadal in the Indian Wells final two weeks ago.

Roddick said his injury occurred while lunging for a shot in the fifth game, and 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 April 6-8 against Spain.

“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.”

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

“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 — or even look at the ball.

He then conceded with a handshake.

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

Roddick’s chances of winning his second Key Biscayne title seemed good when chief nemesis Roger Federer lost Tuesday to Guillermo Canas. Instead, Roddick retired from the tournament for the second time in three years.

A wrist injury forced him to quit in his opening match in 2005.

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 this year with 23 wins — two against Roddick — in 27 matches.

Djokovic had only 17 unforced errors despite taking big swings as he and Nadal waged a series of grinding baseline rallies. Djokovic erased three break points in the final game, then closed out the victory with an ace and collapsed to the court, exhausted but jubilant.

“He played very good,” Nadal said. “I didn’t play the same level as at Indian Wells. The biggest problem was my backhand was not working very well.”

One day after routing Maria Sharapova, Williams won her first set in 25 minutes but struggled through much of the second. She trailed Vaidisova 3-4, love-40, then won 13 of the final 14 points, including aces on the final two points.

“I just went erratic,” Williams said. “I started missing shots and made a lot of unforced errors. My ball started flying. There are no excuses. I just starting playing crazy.”

The victory means Williams will climb into the top 15 in next week’s rankings. She started the year 95th.

Peer advanced to the semifinals by winning the first nine games and beating No. 24 Tathiana Garbin 6-0, 6-3.

Peer is 0-2 against Williams, but was two points from victory when they met in Melbourne.

“What a match I had down there with her,” Williams said. “She has made a lot of great progress.”

The other women’s semifinal Thursday will be between top-ranked Justine Henin and Anna Chakvetadze. With second-ranked Sharapova’s loss Tuesday, Henin is assured of remaining No. 1 next week.

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