Former world number one ,Williams, twisted her left ankle late in her second round match against Serbia’s Bojana Jovanovski and needed about 10 minutes of treatment including having her ankle restrapped, before she went on to secure a 6-2 6-4 victory.
The American is due to meet Slovakia’s Daniela Hantuchova in the quarter-finals and is hopeful she will be able to continue in the tournament.
The injury, however, will create some doubt as to whether the 30-year-old will be in top form as she attempts to win her sixth Australian Open title at Melbourne Park later this month.
“I’m going to have (an MRI and an X-ray) and just kind of see and make sure its OK. I’m anticipating it’ll be OK,” Williams told reporters.
“I’m always thinking I can play on but at the same time I don’t want to stress it out right now, so I’m kind of just going to play it by ear and see how I feel when I wake up in the morning.
“If my movement is not right it’s going to be difficult. Daniela is a good player.”
Stosur, who beat Williams in the U.S. Open final last year to claim her maiden grand slam title, continued her poor run of form in her hometown losing 6-4 6-2 to Czech Iveta Benesova.
It was the second successive year the world number six has been the top seed at Brisbane and lost in the second round. Last year, she lost to compatriot Jarmila Gajdosova.
“Hopefully it was just a bad day and I will bounce back next week and the week after,” Stosur said. “Of course it’s disappointing. I never wanted to go out in this round again. I don’t want to dwell on it.
“It’s back to the practise court and keep working hard and do whatever I can to set up myself up for the summer.
“It’s not the ideal start but I am not going to panic and think it’s all lost.”
Benesova, who had never beaten Stosur in their previous four meetings, will now meet Australian Open champion Kim Clijsters in the quarter-finals.
Men’s second seed Gilles Simon had no problems dispatching Australia’s James Duckworth 6-3 7-5 to move on to the quarter-finals.
The Frenchman joins Uzbekistan’s Denis Istomin in the last eight after he advanced with a walkover following the withdrawal of Tommy Haas with a calf injury.
Men’s top seed Andy Murray meets Gilles Muller later on Wednesday.
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