Serena withdraws with injury

SAN DIEGO, July 30 (UPI) -- Top-seeded Serena Williams withdrew from the Acura Classic in California Friday because of a knee problem.

Williams had been scheduled to play Russian Vera Zvonareva, who advanced to the semifinals after the American pulled out.

The 22-year-old Williams said she had an inflamed right knee.

Williams' sister Venus withdrew earlier in the week with a wrist injury.