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Serena Williams has withdrawn from the USA Olympic tennis team due to a lingering knee injury.
Williams, who has expressed security concerns about the Games, has been struggling recently with the problem which forced her to pull out of tournaments in San Diego and Montreal.
United States Tennis Association spokesman Randy Walker said doctors had "advised her that if she were to play, she'd risk serious long-term repercussions on her knee".
Williams had been hoping to add another gold medal to her collection, having triumphed with sister Venus in the women's doubles four years ago.
She was also entered to play doubles with Venus this year, but the older sister will now play alongside Chanda Rubin.
The former world number one's place in the singles draw will be taken by Australian Samantha Stosur.