Former world number one Serena Williams has pulled out of the Cincinnati Open with a toe injury, tournament organisers said on Wednesday.
Williams was due to face Australian 10th seed Samantha Stosur in the second round, in a rematch of the Toronto Cup final she won on Sunday.
The 13-times Grand Slam champion suffered an "aggravation of a right toe injury" a statement from the organisers said.
Williams has won her last two tournaments since returning to competition in June after 11 months out with injuries and health problems.
After her straight sets victory over Czech Lucie Hradecka on Tuesday, Williams said she was wary of becoming tired before the US Open later this month.
"I definitely don't want to overdo it. My main goal right now is obviously to do well," said Williams.
"And nothing against Cincinnati or Toronto or the Stanfords that I played, but this is all preparation. Everyone is preparing for the US Open. So I have to be smart and make sure it's not too much."
Azarenka pulls out of Cincinnati with hand strain
World number three Victoria Azarenka pulled out of the Cincinnati Open on Wednesday due to a "right hand strain".
The Belarussian announced she was withdrawing shortly before her second round match against Israeli Shahar Peer.
Azarenka's pull out follows Serena Williams' withdrawal earlier on Wednesday due to a toe injury.
Pauline Parmentier of France took Azarenka's place against Peer.