Former US Open champion Samantha Stosur has been beaten in straight sets by 26th seed Flavia Pennetta in the fourth round at Flushing Meadows.

The Italian broke the 2011 winner midway through the first set, testing her opponent with a string of forehand winners after struggling with her own serve a couple of games earlier.

Pennetta struggled while trying to serve the set out, giving the Australian a chance to get back into the opener, but a wayward backhand from Stosur gave her opponent a second chance, which she took.

Stosur came under pressure again early in the second set, battling to hold her first service game, before a double fault in her next gave Pennetta a break point, but the 22nd seed held on.

Double faults continued to plague her though, allowing Pennetta to finally get the break on the board, before being taken to deuce in her next service game while trying to stay in the match.

Stosur forced the 26th seed to serve it out, but Pennetta held with ease to seal her spot in the last eight, where she will face either fifth seed Petra Kvitova or British qualifier Johanna Konta.