Beijing: World number one and current China Open champion Serena Williams withdrew from the tournament Friday because of a left knee injury, organisers told AFP, the second consecutive tournament she has withdrawn from early.
The top seed in Beijing was due to play Australian Samantha Stosur in her quarter final match Friday after winning her third round match Thursday evening.
The power-hitting American made it through after a scare in a back-and-forth game against Czech player Lucie Safarova that she eventually won 6-1, 1-6, 6-2.
Stosur, the 2011 US Open winner and former world number four, will now go straight into the semi-finals.
Williams had to be helped from the court as she retired ill during her first set at the inaugural Wuhan Open in China last week.
She will hold a press conference in Beijing to explain her withdrawal at 1:00 pm local time (0500 GMT).
Her sister Venus, the 16th seed in Beijing, withdrew from the tournament with a viral illness on Thursday.
The $5.4 million China Open will reach its climax on October 5.