Beijing, Beijing, China, 3 October 2014 | AP

Sam Stosur has moved into the semifinals 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 quarterfinals 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.

Stosur will next face No.3 seed Petra Kvitova, who beat Italy’s Roberta Vinci in straight sets.

Kvitova holds a 6-1 head-to-head advantage over the Aussie and has won their last two matches.