NEW YORK (AP) — Garbine Muguruza did not like how her topsy-turvy fourth-round match at the U.S. Open against Barbora Krejcikova ended. And that was not just because Muguruza lost.

It also had to do with what Krejcikova did toward the end of her 6-3, 7-6 (4) victory that finished after 1 a.m. as Sunday became Monday — behavior that two-time major title winner Muguruza told the reigning French Open champion was "so unprofessional" during a brief-as-can-be handshake at the net.

Told by a reporter afterward that she seemed upset, two-time major champion Muguruza replied: "Well, I think, between players, you know a little bit how to behave in certain moments and, yeah, I wasn't very happy at the end of the match."

The eighth-seeded Krejcikova took a medical timeout while down 6-5 in the second set after saying she wanted to see the trainer. When play resumed after the break, Krejcikova won seven points in a row. She also angered the ninth-seeded Muguruza by taking her time between points down the stretch, slowly walking over to her towel.

When the match was done, Krejcikova stayed in her sideline chair, hunched over, until eventually leaving the court with some assistance and looking wobbly.

She did not appear at a post-match news conference. In comments distributed by the U.S. Tennis Association, Krejcikova said: "At the end, I was really struggling and I feel really bad right now. I don't really know what happened, but I couldn't breathe. I started to feel dizzy, and the whole world was shaking. It never happened to me before."

Krejcikova is headed to the U.S. Open quarterfinals in her singles debut at the hard-court Grand Slam tournament.

Krejcikova had participated in the main draw of singles at only four Slams — two each at the Australian Open and French Open — before collecting the title at Roland Garros in June. She also won her third Grand Slam doubles trophy in Paris.

Krejcikova then got to the fourth round in her first singles appearance at Wimbledon in July, and added this run at Flushing Meadows.

The 25-year-old from the Czech Republic plays No. 2 seed Aryna Sabalenka on Tuesday for a spot in the semifinals.

Muguruza won the French Open in 2016 and Wimbledon in 2017, and she was the runner-up at the Australian Open last year. But she has never made it past the fourth round in nine trips to the U.S. Open.

___

More AP tennis: https://apnews.com/hub/tennis and https://twitter.com/AP_Sports