WASHINGTON, D.C., USA -- Former World No.9 Andrea Petkovic of Germany reached the quarterfinals at the Citi Open for the second straight year, upsetting reigning US Open champion and No.2 seed Sloane Stephens of the United States, 2-6, 6-4, 6-2.

World No.3 Stephens, who won her first WTA singles title at the Citi Open in 2015, was looking for her fourth consecutive win over Petkovic after losing their first meeting in 2011. But it was Petkovic, currently ranked 91st in the world, who engineered a stunning comeback in one hour and 47 minutes after being down a set and a break.

"I tried to just hang in there, and become more solid," Petkovic told the media, after her comeback win. "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."

Petkovic, the Citi Open runner-up in 2013, ended the match with 20 winners, two fewer than Stephens, but the American finished with 38 unforced errors, eight more than Petkovic's unforced error total. Petkovic converted four of her six break points, and was successful on 55 percent of Stephens' second service points.

Preparation was key for Petkovic. "I definitely decided to come here early, because I know how difficult the conditions in D.C. are," the German said. "I think it didn’t affect me at all, the humidity, the heat today, it was fine."

Petkovic last beat a Top 10 player at Doha in 2016, when she defeated then-No.5 Garbiñe Muguruza. The German will now face either No.6 seed Belinda Bencic of Switzerland or Nao Hibino of Japan in the quarterfinals, as Petkovic hopes to defend her Citi Open semifinal showing from last year.

The first set was dominated by the American. Stephens broke Petkovic on the fifth point of the match after a service return clipped the net and dropped over on Petkovic’s side for a winner. Stephens kept rolling from there, opening up the court with backhand crosscourt winners as she jumped to a 3-1 lead.

Stephens claimed a second break for good measure, after Petkovic pushed a forehand long while down break point to hand over a 4-1 advantage to the American. Stephens then held for 5-1 at love, when a forehand again rolled over the netcord for an unreachable winner.

Stephens closed out the set two games later, having converted both of her break points in the match. The American balanced her nine unforced errors with nine winners, while Petkovic had 15 miscues in the opening set, to only seven winners.

A frustrated Petkovic dropped serve in the opening game of the second set, falling further behind. But the German began to find the range on her strong groundstrokes, and broke Stephens for the first time to level the match at 2-2.

From that point forward, the unforced errors by Stephens mounted, while Petkovic became impenetrable on serve. Petkovic hung with the World No.3 all set, and was rewarded at 5-4 when three straight errors by Stephens gave the German triple set point. Stephens hit a backhand into the net on the second chance, and suddenly, the match was tied at one set apiece.

Petkovic held at love with a forehand winner to start off the decider with a bang, as her depth and pace of shot increased, while Stephens, fading down the stretch, continued to misfire in the unpredictably windy conditions. Petkovic cracked the set open at 3-2, reaching break point with a backhand crosscourt pass, and converting the opportunity for a 4-2 lead.

Stephens also faltered in her next service game at 5-2, giving Petkovic triple match point. The German broke at love for the win when a deep return forced a netted error from Stephens. Petkovic won 13 of her 16 first service points in the final set as she galloped to the victory by clinching the final four games of the match.