Serena Williams suffered an injury scare ahead of the Australian Open as an ankle injury forced her to withdraw from the Brisbane International.
The American, 30, fell heavily during a rally when serving for the match against Serbia's Bojana Jovanovski.
She recovered to close out the match 6-2 6-4 but hobbled from the court, and later confirmed she will not play Daniela Hantuchova on Thursday.
Williams had initially been optimistic of being fit for the quarter-final.
"I'm anticipating it'll be OK," said the 13-time Grand Slam champion after her match.
"My initial thoughts [are that] I'm thinking I can play on. But at the same time I don't want to stress it out right now.
"I'm just going to play it by ear and see how I feel when I wake up in the morning."
Williams beat Chanelle Scheepers in round one on Tuesday - her first match since she was against Sam Stosur.
Australian Stosur, the top seed in Brisbane, disappointed home fans with a surprise 6-4 6-2 loss to Czech Iveta Benesova.