NEW YORK -- Sunday night had already passed into Monday morning and Iga Swiatek was still trying to make sense of it.

“I don’t really know what happened with my game,” she told reporters. “I felt no control suddenly. I just have to watch [the tape] and see because, yeah, I didn’t really know why I started making so many mistakes.”

The simple answer was Jelena Ostapenko. The quirky, human thunderbolt blew up the US Open draw with a stunning 3-6, 6-3, 6-1 victory. Not only will that end Swiatek’s 75-week run at No.1, but it reshuffles the deck as we contemplate Tuesday and Wednesday’s compelling quarterfinal matchups.

So, towel off, grab a Labor Day refreshment and let’s ponder the possibilities:

Top half

No.6 Coco Gauff vs. No.20 Jelena Ostapenko

Gauff is the 19-year-old darling of the record crowds here at the USTA Billie Jean King National Tennis Center, but Ostapenko knows a little something about being a 19-year-old phenomenon.

Back in 2017, she was two days past 19 when she won her Grand Slam singles title at Roland Garros. Sunday's win over Swiatek was no fluke; Ostapenko is 4-0 against the three-time French Open champion.

The 26-year-old Latvian ripped seven aces and 31 winners, breaking Swiatek’s serve seven times. She’ll need to be just as sharp against Gauff, who was a 6-3, 3-6, 6-1 winner over wildcard Caroline Wozniacki.

Booking her first US Open quarterfinal completed her set of Grand Slam quarterfinal steak knives. Now Ostapenko has a chance to reach her first major semifinal since 2018 Wimbledon.

“I felt like I was playing a lot of tournaments quite well, but some moments here and there were not working,” Ostapenko told reporters. “I still kept trying and kept working.

“I feel like now my game is getting back in place and I’m playing more free, in a good way, and not thinking too much.”

Gauff knows the scouting report.

“She’s hot or cold, to be honest,” Gauff said. “With Jelena, she's a striker, ball-striker. I lost to her in Australian Open this year. Just staying in the match. I might get some more free points with her, more so than Iga. Maybe not. Maybe she'll hit so many winners.”

Head-to-head: 1-1. Gauff -- then only 15 -- defeated Ostapenko at the 2019 Linz final in three sets for her first WTA Tour title. Ostapenko took down Gauff in the fourth round of the Australian Open back in January, 7-5, 6-3.

More from the US Open:


No.10 Karolina Muchova vs. No.30 Sorana Cirstea

With both players scalding hot, this one feels like a 50-50 ball.

Muchova, enjoying her first visit to the Hologic WTA Tour Top 10, has won 11 of 13 matches on North American hard courts and was a finalist in Cincinnati, falling to Gauff. She defeated Wang Xinyu 6-3, 5-7, 6-1 to advance -- giving her, like Ostapenko, a complete set of major quarterfinals.

“This was the last one that I was missing, so I’m really glad that I made it,” Muchova said. “It feels good, but yeah, I don’t want to stop here.

“[Cirstea is] playing some great tennis. I’ll for sure have to be ready and well prepared to play her and to have a chance in a match. I believe that with my game I can be at the top.”

Montreal: Muchova all-court masterclass vs. Cirstea

The 33-year-old Romanian knocked off No.4-seeded Elena Rybakina in the third round and No.15 Belinda Bencic in the fourth to reach a major quarterfinal for only the second time in her career. The first came 14 years ago at Roland Garros.

In addition to Rybakina, consider the list of players she’s beaten this year on United States hardcourts: Aryna Sabalenka, Marketa Vondrousova, Madison Keys, Caroline Garcia (twice) -- and, interestingly, Muchova.

“We’ve had very tough matches, very tight battles every time we played against each other,” Cirstea said. “Only this year we played three times, so I think we know each other quite well. I’m really expecting a tough one.”

Head-to-head: Muchova leads 3-1. The 2023 results: Dubai (Muchova), Miami (Cirstea) and last month in Montreal, with Muchova winning 7-5, 6-4.