Williams, who won the US Open last month to join Martina Navratilova and Chris Evert on 18 grand slam singles titles, also pulled out of the Wuhan Open in China last week through illness.

She explained that she felt pain during her 6-1 1-6 6-2 win over 13th seed Lucie Safarova on Thursday and has now decided to join her sister Venus in withdrawing from the competition.

“I started to feel pain this week,” she told www.wtatennis.com. “It didn’t feel really well last night at all. I almost didn’t go on the court – I did at the last minute. I think it made it a little worse, to be honest.

“It throbs just sitting, standing. I felt it mostly serving because I’m landing on my left knee. That was really killer for me. Moving was okay – I didn’t do a tremendous amount of moving last night. I just tried to move as little as possible. So yeah, I felt it mostly when I was serving during the match.

“I haven’t had time to get an MRI scan yet, but I’ll do that and see what the problem is.”

Stosur, who won the 2011 US Open, will now face third seed Petra Kvitova in the semis after the Czech took an hour and 35 minutes to dispatch Roberta Vinci 7-6 (7/2) 6-4 on Friday.

Kvitova’s first serve was the difference in a tight contest as she fired down seven aces and proved too much for the Italian in the end.