'Rusty' Serena cruises; Azarenka ousts Mauresmo

The shock loss of her big sister Venus has given Serena Williams added incentive to win this year's Australian Open.

Venus was knocked out of the tournament by unseeded Spaniard Carla Suarez Navarro on Thursday, leaving Serena to shoulder the family's title hopes alone.

Serena avoided a repeat of her sibling's defeat when she eased her way into the fourth round on Saturday with a 6-1, 6-4 win over China's Peng Shuai but said her sister's loss was still on her mind.

"It obviously increases my motivation and my desire to win, hopefully to do better," she said.

Serena's next opponent is Victoria Azarenka of Belarus, a woman eight years her junior but an opponent the American is wary off after she captured her first WTA title in Brisbane this month.

Azarenka knocked out former champion Amelie Mauresmo of France

"I've been watching her. She's been around for a while," Serena said. "Obviously she's young and desperate to win and hungry, all the qualities that it takes to be pretty good."

Azarenka, 19, broke six of Mauresmo's nine service games as she cruised to a 6-4, 6-2 victory in 87 minutes.

Mauresmo, who won the Australian Open in 2006 when her semi-final and final opponents both retired injured, put up a brave fight but struggled to hold her own serve against an in-form opponent 10 years her junior.

Serena, Australian Open champion in 2003, 2005 and 2007, has won each of her three matches this year in straight sets but is still not happy with her form.

She went straight back to the practice courts immediately after her second round win over Argentina's Gisela Dulko and did not have it all her own way against Peng.

After racing through the first set in just 31 minutes, the American quickly found herself 3-1 down in the second before rallying back to win five of the last six games and seal victory.

"It was definitely a lot better than my second round but I'm still trying to work on some things and hoping they'll come together," she said.

"I started making more errors and then I lost my serve a couple times. Then I think I put too much pressure on myself and the next thing I know I was down.

"I hit some wild shots on my backhand so that was kind of crazy, so I'm feeling a little rusty, for whatever reason."