Sloane Stephens was dumped out of the Citi Open in the second round by a resurgent Andrea Petkovic on Wednesday.

The second seed, preparing to defend her US Open title on home soil, slumped to a 2-6 6-4 6-2 defeat against the German, who secured her first top-10 win since 2016.

Petkovic was staring down the barrel after falling a set and a break behind, but Stephens' hefty error count of 38 eventually cost the world number three.

"I tried to just hang in there, and become more solid," said former world number nine Petkovic, who ended a three-match losing streak in her head-to-head with 2015 champion Stephens.

"I think I also found a better rhythm on my serve later on, and it gave me the confidence to just let my arm loose and play more solid, less mistakes."

The weather again wreaked havoc with the schedule in Washington as lightning in the area brought about an early end to play.

But four first-round matches were completed. Magda Linette was a 6-4 6-1 victor over Olivia Rogowska, while Jennifer Brady defeated Natalia Vikhlyantseva 6-1 6-4.

There were also straight-set wins for Zheng Saisai and Ysaline Bonaventure.