Top seed and former champion Sloane Stephens crashed out of the Citi Open in Washington as she suffered a stunning 6-2 7-5 first-round loss to Swede Rebecca Peterson on Tuesday.

Stephens, whose best result this year was a run to the Madrid semi-finals in May, was misfiring from the start and never managed to settle into much of a groove in her first meeting with the Swedish world number 70.

Peterson enjoyed a dream start as she raced out to a 5-2 lead and ultimately took the opening set in 29 minutes when Stephens sent a routine backhand from the baseline into the net.

Stephens, who triumphed here in 2015, started to settle down in the second and led 5-4 but Peterson broke back to get back on serve and never looked back.

Up next for Peterson will be the winner of a Tuesday clash between American Sachia Vickery and Italian Camila Giorgi.

Olympic champion Monica Puig also advanced with a 6-4 6-2 victory over American Allie Kiick and will face Russian Anna Kalinskaya in the second round.

American teenager Cori 'Coco' Gauff, who made a stunning run to the fourth round at Wimbledon and had to win two qualifying matches to reach the main draw in Washington, will face Zarina Diyas of Kazakhstan later on Tuesday.

On the men's side, Russian second seed Karen Khachanov will open the evening session against Frenchman Jo-Wilfried Tsonga in a second-round match followed by a first-round battle between Australian Nick Kyrgios and American Thai-Son Kwiatkowski.