"I wasn't at my best today," the Wimbledon champion said. "There is no specific problem, I just feel a little tired. Now it is time to get ready for the semifinal."

Williams will face Flavia Pennetta in the semifinals. The Italian defeated Ayumi Morita 6-1, 6-2.

Second-seeded Agnes Szavay retired with an injury after splitting the first two sets with Eleni Daniilidou 4-6, 7-5.

The fifth-seeded Greek will play fourth-seeded Maria Kirilenko , who cruised past Catalina Castano 6-3, 6-0.