BRISBANE  - US Open champion Samantha Stosur insists she is still on track for a successful Australian Open despite being stunned in the second round of the Brisbane International on Wednesday.

Iveta Benesova ensured Stosur's miserable run at her home tournament went on when the unseeded Czech defeated the top-seeded Stosur 6-4, 6-2 at the Pat Rafter Arena.

The tournament lost the fourth seed Serena Williams after the American fell heavily while serving for the match against Serbia's Bojana Jovanovski.  The injury left Williams in doubt for the year's opening Grand Slam after she was forced to pull out of the Brisbane International with an ankle injury.

The 13-time Grand Slam winner fell heavily and rolled her ankle during a baseline rally towards the end of her second round win over Serbia's Bojana Jovanovski. Williams, who was serving for the match at the time, was able to continue following treatment and held on to beat Jovanovski 6-2, 6-4.

But the injury-prone player limped from the court and she was later forced to withdraw from her quarter-final against Daniela Hantuchova on medical advice. "I’m disappointed to have to pull out of the Brisbane International," Williams said in a statement. I have been to 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 -- not too many -- and see how I feel. I’m still hopeful of playing the Australian Open."

Williams, who normally wears an ankle guard on her left leg, said she thought "not again" as she felt the ankle give way. "I was like, no way, but as long as I was able to walk it was a little better (than in the past)," she said. "I actually took (the ankle guard) off in the second set because it was painful because I was having another problem with my foot and it was hurting the other problem. I have a brace that I didn't put on today -- I wasn't thinking."

Playing in front of her home crowd for the first time since she won at Flushing Meadows in September, an out-of-sorts Stosur was never in the hunt against the world No.54.

Stosur said the shock loss wouldn't impact her campaign at the Australian Open, which starts in less than two weeks. "It was a bit of a bad day but I'll bounce back next week (in Sydney) and the week after and play a lot better, and keep getting better," she said. "Today was one match and I don't want to dwell on it. I guess it's back to the practice court -- there's no need to panic and think that it's all lost because of today."

Stosur, who was born in Brisbane and grew up at the nearby Gold Coast, also lost in the second round in 2011 and had vowed before the tournament that she was ready to make up for past disappointments. But the home hero never looked comfortable against Benesova and was broken once in the first set and twice in the second in a disappointing display.

"I thought she played quite well but I think 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."

It was Benesova's first win over Stosur in five attempts and the first time she had even taken a set off the Australian.