The men's competition in Beijing also suffered a major setback earlier this week when world number one Novak Djokovic pulled out with a back injury.
World number two Sharapova twisted her ankle against Wimbledon champion Petra Kvitova in the Pan-Pacific Open quarterfinals on Thursday and was forced to quit the Tokyo tournament.
“The good news is the MRI scan showed no big damage to my ankle. I will do my best to recover in time for Istanbul (the WTA season-ending championship)," the Russian said on her website (www.mariasharapova.com).
Former world number one Williams has pulled out of the Beijing event for the second year in a row.
The tournament website (www.chinaopen.com) said the American was due to arrive in the city on Friday but failed to show up.
Her absence means neither of the crowd-pulling Williams sisters will be in attendance after Venus pulled out last month.
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