Venus Williams, scheduled to play sister Serena in Thursday's Masters Series semifinals, withdrew at the last moment because of tendinitis in her right knee.
On a weird day, the tournament also lost the world's top-ranked players, Gustavo Kuerten and Martina Hingis, to upsets.
The announcement of Williams' withdrawal, made just as she and Serena were to take the court for their evening match, drew loud booing from the crowd of some 10,000 in the main stadium.
Venus Williams had shown no signs of the injury when she beat Elena Dementieva 6-0, 6-3 in Wednesday's quarterfinals. She said the knee began bothering her warming up for the semifinal and "I was moving like a grandma."
"I made every effort to be on the court, stretching, warming up and taping, seeing the doctor," said Williams, who said she decided she couldn't play about 10-15 minutes before the match was to begin.
"I don't like to play with pain because it can make it a lot worse," she said.
Asked if she would have played the semifinal had it not been against her sister, Venus replied, "No, I don't think so."
She limped slightly as she left the interview room.
Serena plays Kim Clijsters, who upset Hingis 6-2, 2-6, 6-1, in Saturday's final.
Clijsters, who has won three times on the women's tour since turning pro in 1999, consistently drove low, hard groundstrokes down the lines against Hingis.
She hit 21 winners _ 13 on forehands _ to Hingis' 10.
"That's one of the top 10 players I see in the future, for sure," Hingis said. "She basically didn't really have a weakness today."
Clijsters, 17, was thrilled to beat Hingis for the first time in four matches.
"This is probably my greatest tournament, my greatest victory," she said, adding that she was careful not to get too giddy.
"I have to keep my feet on the ground. I'm looking forward to my next match. I know it's going to be very, very hard," she said.
In the men's draw, Jan-Michael Gambill stunned Kuerten 7-6 (7-5), 6-4 to move into the quarterfinals.
Fourth-seeded Andre Agassi outlasted Nicolas Kiefer 6-4, 5-7, 6-4 to also move into the quarterfinals, and Patrick Rafter beat defending champion Alex Corretja 4-6, 6-3, 6-4.