Defending champion Sam Stosur progressed to the fourth round of the US Open but ninth seed Li Na became the latest victim of teenage giant killer Laura Robson.

The 18-year-old from Great Britain followed up her victory over Kim Clijsters in round two, which sent the Belgian into retirement, by upsetting another former grand slam champion.

Li, who won the French Open last summer, looked favourite to come through when she recovered from losing the opening set to level but Robson did not waver and deservedly clinched a 6-4 6-7 (5/7) 6-2 win.

It was a hugely disappointing loss for Li, who has been one of the form players on the north American hard courts, and she felt she had contributed to her downfall.

She said of Robson: "She has a huge first serve. But it's not like didn't have a chance. I was making a lot of mistakes in the whole match, so I gave her a lot of free points. That made her like, 'Oh, I've got more confidence. Oh, I can beat her."'

In the next round Robson will play another of last year's grand slam winners, Stosur, who was given a tough time in the first set today by America's Varvara Lepchenko before easing away to win 7-6 (7/5) 6-2.

Marion Bartoli, the 11th seed, is also through to the last 16 after a 6-2 6-4 victory over fellow Frenchwoman Kristina Mladenovic.