Defending champion Iga Swiatek was sent crashing out of the US Open on Sunday, losing in three sets to Latvia's 20th seed Jelena Ostapenko.
Ostapenko fought back after losing the opening set to record a 3-6, 6-3, 6-1 victory in 1hr 48min and will now face American sixth seed Coco Gauff in the quarter-finals on Tuesday.
The defeat means Polish star Swiatek will end her 75-week reign as women's world number one when the rankings are updated after the US Open, with rival Aryna Sabalenka set to take over top spot.
Ostapenko's victory also extends her unbeaten record against Swiatek after wins in all three of her previous meetings with her.
"I always expect a tough battle against Iga, she's such a great player and won many Slams and is so consistent," Ostapenko said after her win on the Arthur Ashe Stadium court.
"I knew I had to be aggressive and play my game because that's what she doesn't like.
"I was just thinking that I have to play until the very last point, until we shake hands. I felt like I was playing better and didn't give her many chances."
Swiatek had cruised into the fourth round after not dropping a set in her first three matches, looking in prime form to defend the title she won last year.
On Sunday she finally looked ready to score a first victory over Ostapenko, playing confidently to take the first set.
But Ostapenko snatched an early break in the second set and then held for a 3-0 lead and Swiatek was always struggling thereafter.
She managed to break Ostapenko and served to level at 4-4, but instead was broken again to give the Latvian a 5-3 lead.
Ostapenko then survived a break point to close out the set at 6-3.
In the third, Swiatek had no answer to Ostapenko as she broke the champion twice in the first three games to take 3-0 lead.
She then broke Swiatek in the fifth game to go 5-0 ahead.
Although Ostapenko was broken as she served for the match, Swiatek again struggled on serve and was broken again to go tumbling out.
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