Iga Swiatek's defence of the US Open title was ended by an inspired performance from the huge-hitting Jelena Ostapenko in the last 16.
Polish top seed Swiatek lost 3-6 6-3 6-1 to Latvian 20th seed Ostapenko during Sunday's night session in New York.
With her fearless and aggressive approach, Ostapenko hit 31 winners and forced Swiatek into a host of mistakes.
Ostapenko, who won her sole major at the 2017 French Open, will face American sixth seed Coco Gauff next.
Swiatek's 75-week streak as the women's world number one has been ended by the defeat, with Belarus' Aryna Sabalenka guaranteed to replace her after the US Open.
"I'm surprised that my level changed so drastically," said 22-year-old Swiatek.
"Usually when I play bad, I play bad at the beginning, then I catch up or problem solve. This time it was totally the opposite.
"I don't really know what happened with my game. I felt no control suddenly."
Swiatek had lost all three of her previous meetings against Ostapenko, but the pair had not played since February 2022 - after which the Pole embarked on a transformational 37-match winning run.
The run led to Swiatek taking over at the top of the WTA rankings, a place she had not relinquished since as her dominance increased.
How she could cope with 26-year-old Ostapenko's powerful hitting was a key question in an intriguing match-up.
"I always expect a tough battle against Iga, she is such a great player and very consistent," said Ostapenko.
"But I just went for it. I know I had to play my game and be aggressive because that is not what she likes."
After falling an early break behind, Swiatek began to soak up the groundstrokes and quickly turned an entertaining set around.
But she lost serve again early in the second set and this time was unable to rescue the situation.
Hurried by the pace of Ostapenko's returns, Swiatek continued to make errors in the rallies and quickly became completely overwhelmed in an extraordinary deciding set.
Ostapenko landed a flurry of winners as she moved 5-0 ahead and had the opportunity to inflict a rare 6-0 scoreline on Swiatek, who has become known herself for winning 'bagels' against her opponents.
While Swiatek avoided that fate by breaking serve, Ostapenko was not to be denied and fittingly secured victory with another inch-perfect forehand winner to seal a break to love.
On closing out victory, she said: "I was just thinking I have to play until the very last point.
"When I was 5-0 up she was missing more but I thought I was playing better and not giving her any chances."