Madrid:Serena Williams pulled out of the Madrid Open after aggravating a leg injury in a first-round match against Francesca Schiavone.
Second-seeded Williams retired after losing the first set 6-4 to the Italian, citing a problem with her right knee.
Meanwhile, Jo-Wilfried Tsonga of France, seeded ninth, defeated former Wimbledon champion Marat Safin of Russia 6-4, 7-5.
Spain's David Ferrer, seeded 12th, beat Guillermo Canas of Argentina 6-2, 6-2.
A downcast Williams, who had difficulty bending her right knee, said she had made too many commitments with her schedule this year and was paying the price.
"I was just really hindered. My movement was hindered as a result of an injury I've been struggling with for some time," Williams said.
She declined to comment on the extent of the injury, but said it would not keep her out of the upcoming French Open.
"It's not going to stop me playing in Paris," Williams said. "I didn't want to risk my chances to play Roland Garros. I don't know how serious it is but I don't want it to get worse."
Third seeded Elena Dementieva of Russia took a little over an hour to defeat Lourdes Dominguez Lino _the last Spanish woman left in the Open _ 6-3, 6-2.
Tsonga said he was pleased with his attacking game but acknowledged he needed to improve to climb the rankings.
"I played good offensive tennis with some aggressive shots," he said, adding Safin had once been his boyhood idol.
"He played some great matches at Roland Garros and was like an idol of mine. It was special to play against him."
Safin, once the top-ranked player in the world, said after 11 years he was playing his farewell tour and would retire from professional tennis.
"I want to retire while still in decent ranking numbers," Safin said. "I couldn't do anything against Tsonga, I just can't find my game and was left waiting for his mistakes."
Tsonga said the altitude above sea level in Madrid (600 meters) was an advantage for players with big serves.
"The ball travels fast and bounces well, so it is definitely an advantage against players like Rafa Nadal. Rafa is less strong on his serves and returns, but very strong on his shots, so I think playing at altitude is definitely an advantage for us against him," Tsonga said.
After struggling through two injury-hit seasons, two-time Grand Slam champion Amelie Mauresmo of France continued her recent good form, besting Ai Sugiyama of Japan in straight sets.
The 29-year-old 2006 Australian Open and Wimbledon champion beat the Japanese player 6-2, 6-4.
Daniela Hantuchova of Slovakia was taken to a tie-break in the first set by Gisela Dulko of Argentina before taking the second 6-1.
Swiss player Patty Schnyder, who beat Aleksandra Wozniak of Canada 6-2, 6-4, said playing at altitude was not a problem for her.
"Although the court is really fast and the ball bounces high, you can adjust your play by compensating with how taughtly you string your racket," Schnyder said.