Williams twisted her ankle and crashed heavily on the court while serving with a 6-2, 5-3 lead against Serbia’s Bojana Jovanovski in their second-round match Wednesday.

The 13-time Grand Slam winner was able to continue after receiving medical treatment, clinching the match in the next game to secure a quarterfinal spot, but later withdrew after she said tests showed that she had a left ankle sprain “that I probably shouldn’t play on.”

Williams was playing in her second singles match since losing the U.S. Open final in September.

She plans to take a few days off and remains “hopeful of playing the Australian Open,” which begins Jan. 16.