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.
djoker in quarters
Novak Djokovic swept past Croatia's Borna Gojo in straight sets on Sunday to book his place in the quarter-finals of the US Open.
The 23-time Grand Slam singles champion outclassed world No.105 Gojo to win 6-2, 7-5, 6-4 in 2hr 26min on the Arthur Ashe Stadium court.
Djokovic will now face American ninth seed Taylor Fritz on Tuesday for a place in the semi-finals as he chases a record-extending 24th Grand Slam title.
The 36-year-old Serbian star had been given a scare in the third round on Friday, forced to recover from a two-set deficit before beating compatriot Laslo Djere in five sets.
But there never looked like any chance of Gojo causing Djokovic similar difficulties on Sunday, with the Croatian earning just one break point throughout the match.
Djokovic made only 12 unforced errors throughout the match compared to Gojo's tally of 40, and broke his opponent five times on his way to a clinical victory.
"I knew that Borna has a big game...he's a big shot player, serve and forehand, big weapons, and moves pretty well for a big guy," said Djokovic after reaching his 13th US Open quarter-final.
"The key to match was to try and neutralise his serve. Just glad to get through in straight sets."
Djokovic is meanwhile looking forward to the challenge of facing Fritz in the last eight on Tuesday. Fritz is one of three American men in the quarter-finals along with Frances Tiafoe and Ben Shelton, the first time since 2005 three Americans have reached the last eight at the US Open.
"That's amazing for the USTA and US Open," Djokovic said. "All of the guys — Fritz, Shelton and Tiafoe, big serves.
"(Fritz) has been playing some terrific tennis, particularly on home soil here in the States," added Djokovic, who was won all seven of his previous encounters with Fritz.
"Obviously the matches are going to get tougher from here onwards and I'm ready," he added. "It's going to be great."