Samantha Stosur has moved into the semi-finals of the China Open in a walkover after reigning champion Serena Williams withdrew from the tournament with a knee injury.

The top-ranked Williams had been due to face Australian Stosur in the quarter-finals on Friday.

Playing with strapping on her knee, the American beat Lucie Safarova 6-1 1-6 6-2 in the third round on Thursday evening and then skipped her post-match press conference because of swelling in the knee.

Williams retired from the Wuhan Open a week ago during her first-round match with a viral illness.

Her sister, Venus, also withdrew from the China Open this week, citing a virus.