Serena Williams and Ana Ivanovic have become the latest leading players to withdraw from next week's tournament in San Diego, the WTA Tour announced on Wednesday.
It was the fourth successive pullout for world number eight Williams, who injured her left thumb in the fourth-round victory over Daniela Hantuchova at Wimbledon earlier this month.
The American fell in the quarter-finals to top-ranked Justine Henin of Belgium, who withdrew from the San Diego Classic last week with a wrist injury.
Since Wimbledon, the eight-times grand slam champion Williams has pulled out of the Fed Cup semi-final against Russia and tournaments in Cincinnati, Stanford and now San Diego.
The fifth ranked Ivanovic, who reached final of the French Open and semi-finals of Wimbledon, is struggling with a left knee injury. The Serbian also pulled out of this week's Stanford Classic in California.
The San Diego Classic, which will be played for the last time this year, still features world number two Maria Sharapova, number three Jelena Jankovic and Wimbledon champion Venus Williams