Stanford, California:Serena Williams, still hampered by a thumb injury, withdrew from the Bank of the West Classic on Thursday.
Williams has withdrawn from three tournaments since injuring her left thumb and calf at Wimbledon. The event at Stanford begins Monday and will feature 20 of the top 50 players.
"Even without Serena, the depth of this year's field is very strong," said tournament director Adam Barrett. "The overall balance from top-to-bottom should make for some very exciting tennis."
Williams has never won the Bank of the West Classic, while sister Venus Williams has won twice.
Anna Chakvetdatze, the 20-year-old Russian ranked No 8, leads the field. Marion Bartoli, ranked a career-best No 11 after losing to Justine Henin at the Wimbledon final, is also scheduled to play.