Serena Williams has been forced to withdraw from the China Open because of a knee injury.

The world number one has now retired from two consecutive tournaments after pulling out of the Wuhan Open last week through dizziness.

Williams was looking to defend her title in Beijing, and was due to play Samantha Stosur later today in the quarter-finals having battled past Lucie Safarova yesterday.

2011 US Open winner Stosur will now advance straight through to the semis.

Serena's sister Venus also withdrew from the tournament yesterday after being struck down by a viral illness.