Serena Williams injured her left ankle on Wednesday while playing her first tournament since September in what looms as a major setback for her Australian Open preparations.
Williams, in her first trip Down Under since winning the 2010 Australian title and playing a tournament for the first time since losing last year's US Open final, was serving for the match at 6-2, 5-3 against Serbia's Bojana Jovanovski when she was wrong-footed and twisted her ankle, crashing heavily 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 re-taped 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 Brisbane International quarterfinals.
She left the court without commenting and tournament officials said she was being assessed by a doctor. Williams has been troubled by injuries for much of the past two seasons and spent long periods on the sidelines.
Sam Stosur, who beat Williams in the US Open final last September, had an upset 6-4, 6-2 loss to Iveta Benesova in the preceding match on center court, extending a run of never advancing beyond the second round at her home tournament.
It was the 28-year-old Benesova's fourth win over a Top 10 player.
The No 54-ranked Czech player had never taken a set off Stosur in four previous matches but now finds herself in a quarterfinal against Australian Open champion Kim Clijsters.
"Hopefully it was just a bad day and I will bounce back next week and the week after," Stosur said of her Australian Open preparations.
"I don't want to dwell on it.
"It's not the ideal start but I am not going to panic and think it's all lost."
Estonia's Kaia Kanepi beat seventh-seeded Anastasia Pavlyuchenkova of Russia 6-0, 6-3 in another women's match.
On the men's side, second-seeded Gilles Simon of France beat Australian teenager James Duckworth 6-3, 7-5 and sixth-seeded Radek Stepanek of Czech Republic had a 7-6 (3), 6-2 win over Finland's Jarkko Nieminen.