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 Sweden's Rebecca Peterson on Tuesday while teenager Cori "Coco" Gauff also fell in straight sets.
Stephens, whose best result this year was a run to the Madrid semifinals 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 No. 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 set and led 5-4, but Peterson broke back to get back on serve and never looked back.
Up next for Peterson will be Italy's Camila Giorgi, who defeated Sachia Vickery to advance.
Gauff, the 15-year-old sensation who made a stunning run to the fourth round at Wimbledon – and came through qualifiers to reach the main draw in Washington – fell 6-4, 6-2 in the afternoon to Kazakhstan's Zarina Diyas.
The turning point came at the end of the first set as Diyas converted her fourth set point of the game when Gauff sent a forehand into the net.
That brought an end to a game that had lasted more than 10 minutes and left the youngster visibly frustrated.
Diyas held at love to start the second set and rolled to the win before packed stands on a hot and humid day in the U.S. capital.
Gauff did herself no favors, smacking her 33rd unforced error on match point and having also given Diyas 16 break points – five of which she was able to convert.
Baptiste stuns Keys
As one American teenager faltered another delivered as unheralded 17-year-old Hailey Baptiste shocked second-seeded Madison Keys 7-6(4), 6-2 in their first-round meeting.
Keys appeared in control when she went up a break in the first set but Baptiste immediately broke back and managed to outslug the hard-hitting Keys the rest of the way to claim her first career WTA match win.
Olympic champion Monica Puig, fourth seed Hsieh Su-wei and France’s Kristina Mladenovic all advanced in straight sets to the second round on Tuesday.
On the men’s side, France's Jo-Wilfried Tsonga improved his record against Karen Khachanov to 3-0 after coming out on the winning end of a tight 6-4, 2-6, 7-5 battle to send the second-seeded Russian packing.
In the night match, Australia's Nick Kyrgios blasted 15 aces to defeat qualifier Thai-Son Kwiatkowski 7-5, 6-4 to set up a second-round showdown with 11th seeded Gilles Simon of France on Tuesday.
Big-serving Marin Cilic and promising newcomers Daniil Medvedev and Kyle Edmund also advanced in straight sets to reach the second round on Tuesday.