Lindsay Davenport's hopes of defending her Bank of the West Classic title were dashed when she was forced to withdraw from the tournament with a back strain.
The top seed was trailing 5-0 in her second-round match against Germany's Anna-Lena Groenefeld when she retired after calling on the trainer.
Groenefeld will now play Nathalie Dechy in the quarter-finals in Stanford.
There were no problems for fourth seed Kim Clijsters as the Belgian swept past Japan's Ai Sugiyama 6-1 6-2.
Clijsters will now play Slovak Daniela Hantuchova, who ended her match against American Meghann Shaughnessy with four aces to win 6-4 3-6 6-1.
Czech Iveta Benesova beat eighth seed Francesca Schiavone of Italy 3-6 6-3 7-6 (7-4) to set up a quarter-final meeting with third seed Patty Schnyder.
Davenport, who had complained of tightness in her back earlier this week, is now a doubt for next week's Acura Classic.
Serena Williams and Maria Sharapova have already pulled out of the event in San Diego.
World number seven Williams withdrew because of the same ankle injury which caused her to miss the Stanford event.
World number two Sharapova, who has not played since she was beaten by Williams' sister Venus in the Wimbledon semi-finals, has a lower back injury.