Serena Williams, women's doubles gold medallist at the Sydney Games, has pulled out of the Athens Olympics.
Williams will not defend her crown
The American was advised to withdraw from the Games rather than risk long-term injury to her left knee which forced her to miss two WTA events.
"I've never been this disappointed in my career," said Williams, who denied the decision had anything to do with security concerns.
"The good news is I don't need surgery. That's the only good news."
"I am sad and disappointed, not only because I am unable to travel to Greece and participate in the Olympics, but also because I gave my word that I would play," added Williams.
"I feel that I am letting down my sister Venus, coach Zina Garrison and the other members of the US team by not participating."
Serena was due to take part in the doubles competition with sister Venus, but the older sister will now be partnered by Chanda Rubin.
"She was ready to fly to Athens, but stopped to see surgeon David Altcheck before heading to the airport," said US Olympic team spokesman Randy Walker.
"He said she would risk injury by competing and advised her to withdraw."
Serena's place in the singles draw will be taken up by Australia's Samantha Stosur.
Williams' withdrawal comes a day after compatriot Jennifer Capriati was forced to pull out of the Athens Games with a hamstring injury.