Serena Williams withdrew from the Athens Olympics Wednesday, becoming the second American woman to bail on the U.S. tennis squad just days before the Games get under way.

On Tuesday Jennifer Capriati, the 1992 Olympic gold medalist in singles, announced she wouldn't be making the trip to Greece, citing a sore hamstring muscle.

The last-minute moves represent a major blow to the United States' hopes of winning their fourth consecutive gold medal in women's singles. Williams's withdrawal was particularly abrupt, coming just over three hours before the U.S. tennis team boarded its plane for Athens. Only last week she assured U.S. coach Zina Garrison that she was fit enough to compete at Athens despite a knee injury that forced her out tournaments in Montreal and San Diego over the last two weeks.

According to Randy Walker, a spokesman for the U.S. Tennis Association, Williams made a TV appearance in New York Tuesday morning. That was followed by an appointment with David Altchek, a noted knee specialist. "After the visit to the doctor, she made the decision to withdraw," Walker said.

Williams's agent informed USTA officials of her decision by telephone shortly before 4 p.m. The plane carrying the rest of the team departed at 7:05 p.m.

Lisa Raymond, who will represent the United States in doubles with Martina Navratilova, will replace Capriati in singles. Williams's place in the singles draw will be taken by Samantha Stosur of Australia, who is also playing doubles. The world's No. 96 player, Stosur earned the spot by being the highest-ranked player in Athens who wasn't in the singles draw. Williams and sister Venus had planned to defend their 2000 gold medal in doubles at Athens. With Serena out, Venus will team with Chanda Rubin.

Serena underwent surgery on her left knee on Aug. 1, 2003. She returned to the game after an eight-month layoff, but both her caliber of play and commitment to the game have been erratic since. She won her first tournament back, at Key Biscayne, Fla., in April, in blistering fashion. She withdrew from the U.S. Federation's first-round match against Slovenia a few weeks later. At the French Open she was alternately dominant and sloppy, bowing out in the quarterfinals. Then she stormed into the finals of Wimbledon but failed to defend her title, losing in straight sets to Russian teenager Maria Sharapova. Along the way she also expressed misgivings about the level of security in Athens, then retracted those statements, saying in March that "I wouldn't miss this opportunity for anything in the world."

She injured her knee again July 30 during a tournament in San Diego. Last Thursday Garrison said she was confident Williams would compete at the Olympics. "I've actually spoken directly to Serena, and she's fine," Garrison told the Associated Press.

In a statement released Wednesday, Garrison said: "Serena has been battling injuries all year and we, as a team, are disappointed that she will not be able to join us in Athens. We understand that Serena has been undergoing extensive physical therapy since San Diego and that her withdrawal has been based on the advice of her doctors."

Though Venus is the defending Olympic singles champion, she is seeded just sixth. Belgium's Justine Henin-Hardenne, who has been sidelined by a lingering viral illness for months, makes her return to the game as the No. 1 seed. Amelie Mauresmo of France drew the No. 2 seed, followed by French Open champion Anastasia Myskina.

On the men's side, two-time Wimbledon champion Roger Federer of Switzerland is the No. 1 seed. American Andy Roddick, who fell to Federer in a riveting four-set Wimbledon final, is seeded second, followed by Carlos Moya of Spain and Tim Henman of Britain.

The Olympic tennis competition begins Sunday. It will be contested on the same type of hard courts that are used at the U.S. Open, where Roddick is the defending champion.

Serena Williams withdraws from Games because of a knee injury just over 3 hours before U.S. team boarded plane for Athens.