Injuries sideline Serena, del Potro
Williams, a five-time winner of the hard-court event, is out because of the left knee injury that has sidelined the American since her Australian Open title.
Del Potro has a right wrist injury that has bothered him since the start of the season.
The tournament announced their withdrawals Friday, two days after saying Maria Sharapova wouldn't participate because of a bone bruise in her right elbow. Second-ranked Dinara Safina and 2008 champion Nikolay Davydenko also withdrew.
Copyright 2010 by The Associated Press