Serena Williams © Gallo Images

Reigning Australian Open champion Serena Williams defeated China's Peng Shuai 7-5, 6-2 while US fifth seed Venus Williams, the reigning Wimbledon champion, beat Germany's Anna-Lena Groenefeld, 7-5, 6-3.

The sisters could meet in a semifinal at this nine million-dollar event, which saw its greatest upset yet when Australia's 42nd-ranked Samantha Stosur stunned Russian second seed Safina 6-1, 6-4.

"I'm really happy," Stosur said, "I played really well. I executed exactly what I wanted to do."

Safina could have become the women's world number one by winning the title.

"It's very disappointing. That's what I'm playing for," Safina said.

"I want to be there. Suddenly I go to the court and I don't even fight for it. It's just bad behavior from my side. Champions, they don't do this.

"It's a good punishment for me. I have to get smarter."

Serbia's seventh seed Ana Ivanovic, a finalist one week ago at Indian Wells, also was ousted, falling to Hungary's Agnes Szavay 6-4, 4-6, 6-1.

"I'm really disappointed," Ivanovic said. "I hardly did anything well. I couldn't really hold my serve. I had so many chances early on.

I always was missing by a little bit, and got really down on myself."

Szavay, seeded 25th, will next meet 11th seed Victoria Azarenka of Belarus, who ousted Russia's Anna Chakvetadze 6-1, 6-4.

But the stunner of the day belonged to Stosur, who took her first victory over a top-ten opponent since she missed much of the 2007 and 2008 season with Lyme disease.

"It's a great feeling to beat a top-ten player again," Stosur said.

"I came close a few times. Today I got over the line and I'm just really pleased. I knew I could do it. It was just a matter of when it was going to happen."

On the eve of her 25th birthday, the Brisbane native won what she called the biggest victory of her career to book a spot in the fourth round against France's Amelie Mauresmo, who beat Italy's Flavia Pennetta 6-7 (5/7), 6-2, 6-2.

"I've been trying to get a win like this for a long time. I was happy I was able to do it and hopefully there will be more to come," Stosur said.

"When I started playing well again I thought it was possible. Now I'm going to really enjoy it."

Safina plans on a break to try and recapture her hunger for the sport after what she dubbed a disaster.

"It was just a disaster match," Safina said. "She played very good, but from my side, I just can say it was sad the way I played. No excuse from my side. Just basically I wasn't playing."

Russian high seeds Elena Dementieva and Svetlana Kuznetsova advanced. Dementieva, the fourth seed, defeated Spain's Carla Suarez 6-1, 6-2 while number eight Kuznetsova beat Czech Nicole Vaidisova 6-1, 6-4.

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