Czech sixth seed Petra Kvitova is out of the US Open after losing 6-4 6-4 to world number 88 Andrea Petkovic in the second round.
The two-time Wimbledon champion, who has been struggling with a forearm injury, was broken in the 10th game as the German, 31, took the opening set.
She was a break up in the second set before Petkovic broke twice more en route to sealing victory.
Petkovic will now play either Belgian Elise Mertens or Kristyna Pliskova.