Defending champion Svetlana Kuznetsova withdrew on Wednesday from next week's WTA San Diego Open, joining Grand Slam champions Kim Clijsters and Petra Kvitova in pulling out.
The 12th-ranked Kuznetsova, who last played in the third round at Wimbledon, did not reveal the reasons for her late withdrawal, telling her Twitter followers: "Sorry guys, I have to say I won't defend my title in San Diego and very sad about it."
Clijsters, the world number two who is still nursing the vestiges of a June ankle injury, deciding not to test her problem quite so far in advance of the August 29 start of the US Open, where she has won the last two editions.
Wimbledon champion Kvitova told officials this month that she is still suffering with an abdominal injury picked up at the All England club.
Kuznetsova's absence sends 88th-ranked American Vania King into the main draw, which is headed by Russian world number three Vera Zvonareva.