Serena Williamslimped out of the Brisbane International on Wednesday in what could be a major setback for her Australian Open preparations.
The American, playing a tournament for the first time since losing last year's US Open final, was serving for the match with a 6-2, 5-3 lead against Serbia's Bojana Jovanovski when she twisted her ankle and crashed to the court.
The 13-time grand slam winner needed treatment as she lay near the baseline for several minutes and then had the ankle retaped before resuming the match and losing the next point to surrender a service break.
Williams limped through the next game, wincing in pain after at least two points, before securing a 6-2, 6-4 win to advance to the quarter-finals, but was forced to withdraw after a subsequent scan.
"I'm disappointed to have to pull out," she told wtatennis.com.
"I have been to the hospital and it has been confirmed that I have a left ankle sprain that I probably shouldn't play on. I'm going to take a couple of days off and see how I feel. I'm still hopeful of playing the Australian Open."
Samantha Stosur, the top seed and home favourite, was eliminated by Iveta Benesova, with the Czech player triumphing 6-4, 6-2.
"I probably didn't do what I set out to do," Stosur said.
"As the match went on it got harder and harder to try and stop what was happening."
The other women's quarter-finals saw the in-form Kaia Kanepi hammer Anastasia Pavlyuchenkova, the seventh seed, 6-0, 6-3, and Jelena Jankovic, the sixth seed, beat Russia's Nina Bratchikova 6-3, 6-2.
Andy Murray, the men's top seed, was forced to recover from a set down for the second day in succession to beat Luxembourg's Gilles Muller. The world No 4 lost the opening set 6-4 after a dismal final service game but took the second on a tie-break and powered through the decider to win 4-6, 7-6, 6-0. The Scot will next face Cypriot Marcos Baghdatis, a 6-3, 6-4 winner over Kei Nishikori, the fifth seed. Alexander Dolgopolov, the third seed, also faced a stern test before overcoming Russia's Igor Andreev 6-7, 7-6, 6-2.
Gilles Simon beat Sydney teenager James Duckworth 6-3, 7-5 but Bernard Tomic kept home hopes alive with a routine 6-1, 6-2 win over Japan's Tatsuma Ito.