CARSON, Calif. — Serena Williams' injured left knee has knocked her out of another tournament less than three weeks before the Beijing Olympics.

After practicing Tuesday morning, Williams said she knew she couldn't play in the East West Bank Classic. She was scheduled to open Wednesday against Petra Kvitova of the Czech Republic.

"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," Williams said in a statement.

"Unfortunately after hitting this morning, I knew that I wasn't going to be ready for this week. In the meantime, I'm working hard to be ready for the Olympics and U.S. Open."

Williams withdrew from Saturday's semifinals at Stanford after injuring her knee. She said Monday that an MRI exam revealed an inflamed joint, but that she was planning to play Carson against the advice of a doctor and her father.

Last week, she said the injury is different than the one that forced her to undergo surgery on her left knee in 2003 and miss eight months. But that knee has bothered her since, forcing her to drop out of the 2004 Athens Olympics and to miss significant time again in 2006.

Williams said Monday that she was rehabbing the injury three times daily and figured her knee "will be old news" by the time the Olympic tennis competition begins Aug. 10.

Williams is scheduled to play in Beijing, along with her older sister Venus, who earlier withdrew from the Carson tournament because of a right knee injury. They won the doubles gold medal at the 2000 Sydney Games.

Serena Williams, ranked fifth in the world, has played nine tournaments this year and won three consecutive titles. She has a 33-5 match record, including a loss to Venus in the Wimbledon final.

Williams was replaced in this week's draw by lucky loser Melanie South of Britain, who will play Kvitova in the second round.

Besides the Williams sisters, the tournament also lost Lindsay Davenport and Svetlana Kuznetsova to injuries.

Jelena Jankovic of Serbia, the top seed, opens play Wednesday — her first tournament since injuring her right knee at Wimbledon. Jankovic could gain the No. 1 ranking for the first time if she wins the tournament.