Li Na of China reacts to her play against Samantha Stosur of Australia during their third round match of the Cincinnati Open tennis tournament in Cincinnati, Ohio, August 18, 2011.??[Photo/Agencies]

MASON, Ohio - Grand slam title holders Li Na and Petra Kvitova were knocked out of the Cincinnati Open on Thursday while Russia's Maria Sharapova and Vera Zvonareva moved into the quarter-finals.

Australia's Samantha Stosur defeated French Open winner Li for the second successive week with a 6-4 3-6 6-4 win to set up a last eight showdown with Sharapova.

Stosur is hitting some good form in the run up to the U.S. Open, where she reached the quarter-finals last year.

The 10th seeded Australian was runner-up in Toronto last week and with the field wide open in Cincinnati has a real chance at her third career WTA title.

"It would be the biggest title I've ever won," Stosur told reporters. "It would be fantastic but there is still a fair way to go before that can happen.

"I guess making the final last week gives me good confidence going into this week."

Wimbledon champion Kvitova fell to emerging German Andrea Petkovic, ranked 11th in the world, who enjoyed a 6-3 6-3 win in front of a smattering of fans on an outside court.

At the same time, on centre court, Sharapova was making quick work of her compatriot Svetlana Kuznetsova in a 6-2 6-3 victory.

"I felt really good considering I was playing a pretty tough opponent," said Sharapova. "We've had really tough matches in the past, usually three setters. So it was quite nice to get that win in two.

"She's someone who has a tremendous amount of experience and is a really solid player, always dangerous."

World number two Zvonareva crunched Croatian qualifier Petra Martic 6-2 6-2.

Another Russian Nadia Petrova enjoyed a 6-3 6-3 victory over American teenager Christine McHale, who upset world number one Caroline Wozniacki in the second round.