Serena Out of Olympics

Defending Olympic doubles champion Serena Williams has become the latest big name to pull out of the women's tennis tournament in Athens, just four days before competition is due to start.

Organisers gave no reason for her withdrawal but said the world No11 had been carrying a knee injury which forced her to pull out of recent tournaments in San Diego and Montreal.

It is unclear if Venus Williams, who partnered her younger sister to gold in Sydney four years ago, would defend her title with another player or pull out of the event. If she decides to play, it will almost certainly be with Chanda Rubin, who is in Athens for the singles. The US team has until Thursday to make that decision.

Serena's withdrawal comes just a day after fellow American Jennifer Capriati, gold medallist in the singles in Barcelona 12 years ago, said she would not compete in Athens due to a hamstring injury. Capriati was replaced in the US team by Lisa Raymond.

Olympic organisers said Serena's berth in the singles would be taken by Australia's Samantha Stosur, currently ranked 96th in the world.

Venus is still entered in the singles competition, which she won in Sydney in 2000, along with Chanda Rubin and Lisa Raymond.

The US have won the singles at the last three Olympics with Lindsay Davenport capturing the title in 1996 and Capriati in 1992.