Serena Williams has withdrawn from the USA Olympic tennis team.
Williams has been struggling recently with a knee injury that forced her to pull out of tournaments in San Diego and Montreal.
She 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. It is not yet known whether another member of the US squad will now partner her sister.
The former world number one's place in the draw will be taken by Australian Samantha Stosur.
