Serena Williams and Lindsay Davenport have both pulled out of next week's JPMorgan Chase Open in Los Angeles.
Williams and Davenport are lacking preparation for the US Open
Williams remains sidelined with a left ankle injury that has also forced her out of the last two WTA events.
World number one and defending champion Lindsay Davenport withdrew from the event with a back injury.
"I am very disappointed that I have to withdraw and will be unable to defend my title," said Davenport. "My back is just not ready yet for competition."
Serena's sister Venus has withdrawn from next week's other WTA event, the Nordic Light Open in Sweden, because she is suffering from flu.