Second seed Serena Williams has withdrawn from the Los Angeles Classic with a knee injury, the tournament announced on its website on Tuesday.
"After hitting this morning, I knew that I wasn't going to be ready for this week," Williams said in a statement posted on www.eastwestbankclassic.com.
The world number five had been in doubt for the tournament since withdrawing from the semi-finals of the Stanford Classic with an injury to her left knee.
Serena's sister Wimbledon champion Venus, Russian Svetlana Kuznetsova and American Lindsay Davenport also have withdrawn from the tournament because of injuries.
"It's a huge disappointment," Serena said.
"I've been getting intensive therapy and doing everything in my power to get my knee in shape for this week, waiting until the last possible moment to see if I could play," she added.