World number one Serena Williams pulled out of the China Open hours before her quarter-final against Sam Stosur on Friday because of an injury to her left knee.
The American, who won her 18th grand slam singles title at the U.S. Open last month, also withdrew from last week’s Wuhan Open in China because of illness.
The 33-year-old had reached the last eight on the Beijing hard courts after a 6-1 1-6 6-2 win over Czech Lucie Safarova late on Thursday.
No details about the severity of the injury were revealed.
Unseeded Stosur will face either Wimbledon champion Petra Kvitova, who reached the last eight after Serena’s sister Venus pulled out of their third round match with a viral illness, or Italian Roberta Vinci in the last four.
