Iga Swiatek said she was at a loss to describe the way in which her US Open title defence was left in ruins Sunday in a last-16 defeat to Latvia's Jelena Ostapenko.

The Pole lost for the fourth time in as many meetings with the talented but inconsistent Ostapenko, who roared back to clinch a 3-6, 6-3, 6-1 victory after dominating the final two sets.

"She plays well against me. I mean, I don't have any comparison because she always did that," said Swiatek.

"But I'm just surprised that my level changed so drastically because usually when I play bad, I play bad at the beginning, then I kind of catch up or just problem solve. This time it was totally the opposite.

"I don't really know what happened with my game. I felt no control suddenly. I just have to watch and see 'cause, yeah, I didn't really know why I started making so many mistakes."

The 22-year-old Swiatek will lose her status as world number one following the US Open, with Aryna Sabalenka of Belarus set to take over top spot.

Swiatek had cruised into the fourth round in New York after not dropping a set in her first three matches, looking in prime form to defend the title she won last year.

But Sunday's defeat will snap her 75-week reign at the summit.

"Usually I'm not looking at numbers, but overall I love them. I would love to, like, extend this record a little bit longer," said Swiatek, a five-time Grand Slam champion.

"This is something that when I was younger, I actually kind of wanted to break some record or have something. 

"I already did that 'cause I already won a Slam as a first Polish player. Obviously being No. 1 as the third player in history is great.

"But for sure when it happens, when you lose it, there are some sad emotions. 

"All these great players know it's going to come back if you're going to work hard, focus on the right things, just develop as a player."

mw/rcw