Former world number one Serena Williams [Images] was forced to dig deep before beating 11th-seeded Russian Nadia Petrova 6-1, 3-6, 6-3 on Sunday to reach the quarter-finals of the Australian Open.

The seventh seed, winner at Melbourne Park in 2003, dropped her first set of the tournament but held on to set up a quarter-final with second seed Amelie Mauresmo of France [Images].

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

"My serve was horrible. I made errors. I didn't play well or hit my shots right. I'm going to have to improve a lot in my next match or it's going to be a struggle.

"I don't really see too many positives out of the match today except I think I ran well for some drop shots. That's about it."

Williams looked set for a comfortable victory when she raced through the first set in 22 minutes but former French Open semi-finalist Petrova cut out her errors and broke the American twice to level the match.

Williams regained the initiative in the decider, breaking in the opening game on her way to a 4-0 lead and though Petrova hit back for 4-3, the American held on for victory.

Despite her below-par performance, Williams said she was hopeful she would be back to her best against Mauresmo.

"I really like playing her," said Williams, who has won eight of their nine meetings.

"I definitely think we've had some great matches. And she definitely brings out some great shots.

"I'm practising hard... so hopefully it'll come together when I play again."

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