American ace Serena Williams, who won the women's doubles gold medal at the Sydney Olympics, has pulled out of the Athens Games.

No reasons for her withdrawal were given by organisers but they said the former world number one had been suffering from a knee injury.

On Tuesday, former Olympic tennis champion Jennifer Capriati withdrew from the Games, possibly signalling the end of her patchy Olympic career.

The American, 28, did not give a reason for her withdrawal.

Capriati won Olympic gold in Barcelona in 1992 aged just 16 but did not compete in the next two editions of the Games as her career fell apart after she was caught shop-lifting in 1993.

On Wednesday, Capriati said she agonised for days before finally pulling out of the Games with a hamstring injury.

She is replaced in the U.S. team by Lisa Raymond.

"This was such a tough decision, and I am truly disappointed that the strain to my hamstring re-injured last week in Montreal will preclude me from competing for the U.S. Olympic Tennis Team in Athens," Capriati said after pulling out.

"My Olympic experience at the 1992 Games in Barcelona remains one of the most special in my career and I waited until the last minute, hoping I could again represent my country.

"Instead, I will be watching and rooting for the U.S. team to bring back Olympic gold."

The tennis tournament starts on Sunday, two days after the opening ceremony.