Simona Halep is the first big name to be bundled out of the first round of the US Open, with the former world number one beaten by 2019 Wimbledon junior champion Daria Snigur in three sets on the first day in Flushing Meadows.
Making her grand slam debut, Snigur set the tone early against the Romanian seventh seed, storming out to claim the first set 6-2 before Halep fought back to win the second 6-0.
However, the 20-year-old Ukrainian returned fire in the third, taking a 5-1 lead thanks to a double break of serve in the first and fifth games.
While Halep fended off two match points, Snigur held her nerve to defeat last year's Wimbledon semi-finalist 6-4 and claim the biggest win of her fledgling career, bursting into tears and forming a heart shape with her hands around the Ukrainian ribbon pinned to her shirt.
"I'm very happy, it's impossible for me," Snigur said.
"I have to say thanks to Simona for this incredible match. I want to thank all the fans who watched our match today.
"I was very, very, very nervous but I tried to do my best.
"Of course, this match is for Ukraine, for my family, for everyone who supported me. I want to say thank you to you all. Slava Ukraini!"
She will face either Poland's Magdalena Frech or Canada's Rebecca Marino in the second round.
Elsewhere, American Coco Gauff registered a comfortable victory over France's Leolia Jeanjean to claim her first career win at Arthur Ashe Stadium 6-2, 6-3.
Gauff, 18, was runner-up at the French Open earlier this year and is expected to make a deep run at her home tournament.
In round two, she will face the winner of the match between Elena-Gabriela Ruse and Australian Daria Saville.
In the men's draw, Alex De Minaur continued his recent good form to sweep past Serbia's Filip Krajinovic 7-5, 6-2, 6-3, while countryman John Millman was pushed to the limit but ultimately fell to American Emilio Nava 6-7, 6-4, 6-7, 6-1, 1-6.
Nava will take on Andy Murray at the next stage after the Scot eased past Argentina's Francisco Cerundolo in straight sets 7-5, 6-3, 6-3.
World number one Daniil Medvedev also made light work of USA's Stefan Kozlov, taking out his first match 6-2, 6-4, 6-0.
