Injuries sideline Serena, del Potro

KEY BISCAYNE, Fla. -- Add 12-time Grand Slam champion Serena Williams and U.S. Open champion Juan Martin del Potro to the list of top players pulling out of the Sony Ericsson Open.

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.

The 13-day tournament begins Tuesday. The field still includes Roger Federer, Rafael Nadal and Venus Williams.