A glum-faced Serena Williams admitted she deserved to lose after struggling past Nadia Petrova in the fourth round of the Australian Open.

The four women's favourites in the bottom half of the draw all advanced to the last eight but it was Williams, the 2003 Australian Open champion, who was finding it hardest to find any positives after grinding out a 6-1 3-6 6-3 victory over the No.11 seed.

Wimbledon champion Maria Sharapova and US Open titleholder Svetlana Kuznetsova continued to fly the Russian flag for the new superpower of women's tennis, while world No.2 Amelie Mauresmo crushed yet another Russian, Evgenia Linetskaya, 6-2 6-4 to set up a heavyweight quarter-final with Williams.

The American committed 37 unforced errors and only got 53 per cent of her first serves into play against Petrova.

"My serve was horrible and I've been working really hard on it," said Williams.

"It's just that I'm a perfectionist and I played a really bad game today.

"To be honest, I didn't deserve to win the way I played.

"But I guess I have to get over it and play better in the next round."

Pushed to single out some positives, the seventh-seeded Williams could only offer that she had moved well to run down some drop shots.

Williams has won nine of her previous 10 matches against her next opponent Mauresmo, with the only loss coming on clay in Rome two years ago.

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