Defending champion Samantha Stosur has admitted that she expects a difficult match against Laura Robson when they meet in the fourth round of the US Open.

Following her win against Kim Clijsters, 18-year-old Robson caused another shock on Friday when she beat ninth seed Li Na in three sets.

"[Laura's] obviously full of confidence and had two very, very good wins," Stosur said. "Definitely can't go into that match lightly. I have to go out there and be on my game and ready to play some good tennis."

Stosur booked her place in the last 16 with a straight-sets victory over American Varvara Lepchenko.