Former Wimbledon champion Petra Kvitova and reigning U.S. Open titlist Samantha Stosur moved into the fourth round, while former French Open winner Li Na suffered a third-round upset at the final Grand Slam event of the year -- the U.S. Open.

The fifth-seeded left-handed Czech slugger Kvitova handled France's Pauline Parmentier 6-4, 6-4 on the Grandstand court on a very hot day in New York.

Last year's Wimbledon champion Kvitova will lock horns with 2007 Wimbledon runner-up Marion Bartoli on Sunday.

The seventh-seeded Stosur defeated 31st-seeded Uzbekistanian-born American Varvara Lepchenko 7-6 (7-5), 6-2 at Ashe Stadium.

The Australian Stosur, who stunned American great Serena Williams in last year's finale in Flushing Meadows, will battle upstart British teenager Laura Robson in the fourth round.

The ninth-seeded Li was knocked out by the rising Robson 6-4, 6-7 (5-7), 6-2 at Armstrong Stadium. The 18-year-old Robson beat a former major champion for the second time in two matches, as she ended the singles career of three-time U.S. Open winner Kim Clijsters on Wednesday.

The talented Robson, the youngest woman in the top 100, is now into the fourth round at a major for the first time in her career, and she's the first British woman to reach the round of 16 in Flushing in 21 years (Jo Durie, 1991).

Also on Day 5 at the USTA Billie Jean King National Tennis Center, an 11th- seeded Bartoli handled fellow Frenchwoman Kristina Mladenovic 6-2, 6-4.

The 2012 women's singles champion in New York will earn at least $1.9 million. Kvitova could take home $2.9 million as the result of receiving a $1 million bonus for winning the U.S. Open Series.