Second seed Sloane Stephens has suffered a shock 2-6 6-4 6-2 loss to Germany’s Andrea Petkovic in the second round of the Citi Open, disrupting the American’s buildup to her US Open title defence.

Stephens’ exit left the Citi Open without its two top seeds after Caroline Wozniacki pulled out of the tournament on Tuesday with a leg injury.

After a straight sets victory over compatriot Bethanie Mattek-Sands in her opening match, Stephens looked poised for another uncomplicated outing as she strolled through the first set.

Yet the world No.3 ran into considerably more resistance in the second as Petkovic dug in, securing a key break at 5-4 then holding serve to get the match back on level terms.

The 91st-ranked German dictated play in the third set, breaking Stephens twice. She closed out the match with her second break.

“I didn’t serve great, but that’s not what lost me the match,” Stephens said. “She just played better than me.”

Through the first two sets Petkovic faced three break points and could not manage to save a single one, but in the third it was the 30-year-old German applying all the pressure, while not facing a single break point herself.

Petkovic will play the winner of Swiss sixth seed Belinda Bencic’s match against Japan’s Nao Hibino.