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 the match yesterday, Roddick had decided it was time to quit.
He retired with a strained left hamstring, allowing Murray to advance to the semifinals of the
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 tonight when she faces No. 14 Shahar Peer. A 19-year-old corporal in the Israeli Army, Peer nearly pulled off an upset against Williams in the quarterfinals of the Australian Open in January, losing the third set, 8-6.
Roddick said his injury occurred while lunging for 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 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 US Davis Cup team's match against Spain April 6-8.
"It's a hamstring -- it could take three days or three weeks," US captain Pat McEnroe said.
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 -- or even look at the ball.
He then conceded with a handshake.
Murray's opponent tomorrow 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. After last night's loss, Nadal said he will skip Spain's Davis Cup quarterfinal match against the US next month because he needs to rest his injured foot.
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. "There are no excuses. I just started 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.
The other women's semifinal today will be between top-ranked Justine Henin and Anna Chakvetadze.
After her match yesterday, Williams said her boycott of the Indian Wells tournament will continue, despite new WTA Tour rules making participation mandatory.
Williams and her sister Venus last played in the California tournament in 2001, when the family was booed after Venus withdrew just before a semifinal match against Serena. Their father, Richard, said the jeers were racially motivated.
"I can guarantee you the chances of getting me to Indian Wells are slim to none, unless -- I'm not going to go back. I have no interest in going. It's just how I feel."
The new WTA Tour rules announced Tuesday require players to participate in four events -- Indian Wells, Key Biscayne, and new tournaments in Madrid and Beijing.