Flavia Pennetta has caused an upset in the second round of the US Open by knocking out fourth seed Sara Errani.

Both competitors were well matched on the court in the early stages of the contest, but a break of serve proved vital for Pennetta, who fended off Errani's attack to take a one-set lead.

Errani needed to get an early break to get back into the game, but she struggled while Pennetta's shots across the court continued to dominate the clash.

The fourth seed could only muster one game in the second set, with Pennetta eventually winning the game comfortably.