NEW YORK (AFP) — Defending champion Iga Swiatek crashed out of the US Open on Sunday after a defeat by Jelena Ostapenko and will lose her No. 1 ranking, while Novak Djokovic marched into the quarterfinals.

Swiatek grabbed the opening set against Ostapenko but the unpredictable Latvian stormed back to win 3-6, 6-3, 6-1 and secure her fourth victory in as many meetings with the Polish star.

The mercurial Ostapenko, who had never made it to the last 16 in New York before this week, goes on to face title contender and home favorite Coco Gauff in the last eight after blasting Swiatek out of the tournament.

"She plays well against me, she's always done that," Swiatek said.

"I'm just surprised that my level changed so drastically. I don't really know what happened with my game, I felt no control suddenly."

Undeterred by falling behind, Ostapenko's typically fearless and aggressive approach rendered Swiatek helpless and ensures there will be a new World No. 1 after the US Open.

Swiatek's 75-week reign will come to an end, with rival Aryna Sabalenka set to take over top spot.

"I always expect a tough battle against Iga, she's such a great player and won many Slams and is so consistent," said Ostapenko, the 20th seed and 2017 French Open champion.

"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."

Gauff ended Caroline Wozniacki's fairytale Grand Slam comeback earlier Sunday as the sixth seed saw off the former world number one 6-3, 3-6, 6-1.

Djokovic swept into his 13th US Open quarter-final, cruising past 105th-ranked Croatian qualifier Borna Gojo 6-2, 7-5, 6-4 to advance to a showdown with American No.1 Taylor Fritz.