Williams, the winner of eight grand slam titles, injured her thumb during her run to the quarter-finals at Wimbledon and has since pulled out of the Fed Cup and Cincinnati Open.
French Open runner-up Ana Ivanovic of Serbia also withdrew with a knee injury.
The field will feature eighth ranked Anna Chakvetadze of Russia, Wimbledon finalist Marion Bartoli of France, Switzerland's Patty Schnyder and Daniela Hantuchova.
The following week's tier I tournament in San Diego has also been struck by pullouts. World number one Justine Henin announced on her Web site on Wednesday that she cannot play due to a wrist injury, while 10th-ranked Nicole Vaidisova withdrew with a viral infection.
