British No 2 Harriet Dart landed the biggest win of her career by dumping out 10th seed Daria Kasatkina, while seventh seed Simona Halep was knocked out by Ukrainian qualifier Daria Snigur.
Dart had never previously won a main draw match at the US Open and looked to have a very tough task on her hands against Russian Kasatkina, who is ranked ninth after winning a WTA Tour title in Granby on Saturday.
But Dart battled superbly in hot conditions on Court 12, recovering from a shaky second set and then coming from behind in the third to claim a sensational 7-6 (10-8) 1-6 6-3 victory.
Dart, 26, is enjoying the best season of her career having made the jump into the top 100 and picked up some eye-catching wins, including over Elina Svitolina on her way to the fourth round in Indian Wells.
She also had a strong grass-court season but the Londoner had never previously defeated a top-10 player.
There were 10 breaks of serve in the opening set, which Dart edged on a tie-break after saving two set points.
Kasatkina, who reached the French Open semi-finals earlier this year, hit back in the second set, pushing Dart well behind the baseline, and looked in control early in the third.
But Dart kept fighting and earned her reward, holding her arms aloft after clinching her first match point to set up a second-round clash with Hungary’s Dalma Galfi.
Halep stunned by qualifier
Snigur, who was Wimbledon girls’ champion in 2019, wore a ribbon in the colours of the Ukrainian flag on her chest and made a heart shape around it at the end of her 6-2 0-6 6-4 victory.
Halep went into the event as one of the favourites after winning the big warm-up title in Toronto and returning to the top 10 but, although she threatened a comeback from 5-1 down in the decider, Snigur made it over the line.
“I’m very, very, very nervous but I tried to do my best,” she said in an on-court interview. “For Ukraine, for my family, I want to say thank you, all.”
Snigur’s father is with her in New York but her mother is at home in Kyiv, where the youngster is no longer able to train after the centre where she trained was bombed in the war.
“The situation is bad, of course,” she said. “I try to do the best for Ukraine, I try to support my country. It’s not so easy because the war is continuing. Sometimes it’s impossible to play but I try to do my best because I want to live in Ukraine.
“Before the war I practised in Kyiv but Russia bombed my base so before the US Open I practised in Riga with my coach but I don’t have a base where I can practise every time.
“I think (Ukrainian) people enjoyed my match because I have many messages. I didn’t understand what happened. I will try to understand it tomorrow because it was I think the best match in my career.”
Gauff wins on Ashe to move into second round
American Coco Gauff eased into the second round with a comfortable 6-2 6-3 victory over France’s Leolia Jeanjean.
Gauff, the 12th seed and home favourite, had the crowd at Arthur Ashe Stadium firmly behind her as she wrapped up the match in 79 minutes on the back of a strong first serve.
“I’m super excited because this is my first win on Ashe. I think this is my fourth match here on this court and it’s my first win,” she said.
The 18-year-old, who was French Open runner-up earlier this year, fired eight aces and lost only two points on her first serve.
Third seed Maria Sakkari gained revenge for her Wimbledon defeat by Tatjana Maria with a 6-4 3-6 6-0 victory.
Former champion Bianca Andreescu saw off Harmony Tan 6-0 3-6 6-1 and Wimbledon finalist Ons Jabeur began her campaign by beating Madison Brengle 7-5 6-2.