Last year's U.S. Open champion was serving for the match at 6-5 and 30-30 in the second set when the chair umpire failed to notice an obvious let call when Williams' serve hit the net and into the serving area. Williams didn't even bother to chase down Stosur's return, believing it to be a certain let.
Stosur was awarded the point and went on to break Williams and won the ensuing tiebreaker. Williams fought off four match points in the 10th game of the third set before breaking Stosur two games later to win the match.
Until her late rally, Williams seemed visibly upset by the mistake. She spoke to the umpire, Asitha Attigala of Sri Lanka, at the end of the game and second set, but didn't argue. At the end of the match, she shook his hand.
Williams acknowledged the incident had disrupted her game.
"I felt like I was going to win that second set,'' she said after the match. "And then I hit a massive net cord and the guy didn't call it. Even Sam knew it was a net cord. It was frustrating at that point. These types of things seem to happen to me a lot.''
The heavily pro-Stosur crowd at the Olympic Park Tennis Centre - site of the 2000 Olympics tennis competition - loudly cheered every Stosur point. Williams received only smatters of appreciative applause even on her big points.
Williams said she was disappointed in her game, noting that she had missed a lot of easy shots and that the strong wind had sometimes been challenging.
"It was definitely tough conditions with the wind,'' Williams said. "With that being said, I just made so many errors, and that was a little frustrating because I feel like I've been working hard and to come out and perform at the level I did, I wasn't extremely happy.''
The last time the two played, Williams beat Stosur in straight sets at the Beijing Olympics in August, when Stosur only won two games.
Stosur said she should have won the match but was pleased that she had done better against Williams than ever before despite "a litle bit of extra nerves'' during some of the later serves.
"I'm very disappointed at the moment but I'm trying to look at the positives,'' she said. "I could have went down in straight sets but I kept going and then found myself at four match points.''
Williams, ranked No. 2 and last year's U.S. Open winner, pulled out of the WTA Championships last November and later from the U.S. team for the Hopman Cup in Perth, Australia, due to a lingering hamstring injury. She arrived in Australia this week saying she was injury-free.
"I'm feeling good. I've had some extra time to get really fit,'' she said Sunday. "I'm ready for the Australian Open.''
In another match Sunday, former U.S. Open champion Svetlana Kuznetsova of Russia, the fifth seed here, also needed three sets to beat China's Peng Shuai, advancing to the second round with a 3-6, 6-1, 6-3 win.
Second-seeded Dinara Safina of Russia, a losing finalist at the Hopman Cup team event with her brother Marat Safin against Slovakia, plays Sorana Cirstea of Romania in the first round Monday.
In the joint ATP-WTA tournament, top-seeded Novak Djokovic and third-seeded Jo-Wilfried Tsonga both have received first-round men's byes and could meet in the semifinals. Djokovic beat Tsonga in last year's Australian Open final.
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