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 while teenager Cori 'Coco' Gauff also fell in straight sets.
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 Italian 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 Zarina Diyas of Kazakhstan.
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 US capital.
Gauff did herself no favours, smacking her 33rd unforced error on match point having also given Diyas 16 break points, five of which she was able to convert.
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 Frenchman 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 Australian 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.
Azarenka stomps Tan at Silicon Valley Classic
Former world number one Victoria Azarenka overpowered tournament qualifier Harmony Tan 6-2, 6-4 to reach the second round of the Silicon Valley Classic in Northern California on Tuesday.
The Belarusian converted seven of her 12 break point opportunities to advance to the second round, where she will face fifth-seeded Donna Vekic of Croatia.
American Danielle Collins came out ahead in a marathon 6-4 5-7 6-2 battle with China’s Zhang Shuai on a sun-soaked afternoon on the campus of San JoseStateUniversity.
The rising 25-year-old University of Virginia product said her conditioning was the difference in her first-round match.
“I think I did a really great job making it physical,” Collins said.
“I’ve been working a lot on my fitness the last couple weeks after Wimbledon. I’ve had a good couple weeks of training and I think it paid off today.”
Collins will next face Zheng Saisai after the Chinese defeated Britton Heather Watson 6-3, 6-1 in their first-round match.
