Li Na of China leaves the court after her loss to Laura Robson of Britain in their women's singles match at the US Open tennis tournament in New York August 31, 2012. [Photo/Agencies]

FLUSHING MEADOWS, New York - British teen Laura Robson stunned Chinese ninth seed Li Na at the US Open on Friday to book a fourth-round showdown against defending champion Samantha Stosur.

Robson, an 18-year-old left-hander who ended the career of three-time US Open champion Kim Clijsters with a second-round triumph, took her first victory over a top-10 opponent by downing 2011 French Open winner Li Na 6-4, 6-7 (5), 6-2.

Australian seventh-seed Stosur, who has not won a title since lifting the US Open trophy last year, reached the fourth round with a 7-6 (5), 6-2 victory over American 31st seed Varvara Lepchenko.

"She's already had two very good wins and she's full of confidence," Stosur said. "I'm going have to be ready to play my best game if I'm going to beat her.

"It can be a very tricky match. She's got nothing to lose and she has had two of the biggest wins of her career."

Robson, who was born in Australia but moved to Britain at age six, became the first British woman in the fourth round of a Grand Slam since Sam Smith at Wimbledon in 1998 and the first in the Open's last 16 since Jo Durie in 1991.

Uzbek-born lefthander Lepchenko, who became a US citizen a year ago, has still never beaten a top-10 player. She was trying to match her best Grand Slam result, this year's fourth-round French Open run.

After fighting to 5-5 in the tie-breaker, Lepchenko sent a lob and a backhand wide to surrender the opening set after 59 minutes. Stosur broke in the third and fifth games of the second set and held to finish in 88 minutes.

French 11th seed Marion Bartoli also advanced with a 6-2, 6-4 victory over compatriot Kristina Mladenovic, setting up a last 16 match against fifth-seeded Czech Petra Kvitova, who defeated Pauline Parmentier of France 6-4, 6-4 to reach the fourth round.

Third-seeded Russian Maria Sharapova, the reigning French Open champion, meets US wildcard Mallory Burdette and Belarus top seed Victoria Azarenka, the reigning Australian Open champion, plays China's Zheng Jie in later matches.