WASHINGTON (REUTERS) - Second seed Sloane Stephens was shocked 2-6 6-4 6-2 by Germany's Andrea Petkovic in the second round of the Citi Open on Wednesday (Aug 1), disrupting the American's buildup to her US Open title defence.

Stephens' exit left the Citi Open without its two main draws and top seeds after Caroline Wozniacki pulled out of the Washington, DC tournament on Tuesday with a leg injury.

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

Yet the world number three 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.

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.