WTA China Open quarter-final against
Samantha Stosur due to a knee injury.
Williams - defending champion at the WTA
Premier tournament in Beijing - would have
sealed the world number one ranking for the
year's end with victory in China.
But a left knee problem has dashed those hopes
for at least another week, with Australian Stosur
into the last four as a result of the American's
withdrawal on Friday.
Stosur has enjoyed an excellent week at the China
Open, recording straight-sets triumphs over
Francesca Schiavone, Caroline Wozniacki and
Alize Cornet.
The 30-year-old - who won the US Open in 2011 -
has not won a Tour title in 2014.
Williams fell ill during last week's Wuhan Open
and had to be helped from the court after retiring
against Alize Cornet in the second round.
Serena's older sister, Venus, also pulled out of the
China Open, citing a viral illness on Thursday.
Stosur will play Petra Kvitova of the Czech
Republic in her semi-final. Kvitova beat Italian
Roberta Vinci 7-6 (7-2) 6-4 on Friday.
Sent from my BlackBerry® wireless handheld from Glo Mobile.