This U.S. Open has become “The Late Show with Madison Keys.”

Keys made it through to the quarterfinals late on Monday night with a stirring three-set victory over fourth-seeded Elina Svitolina in a match that ended around 11:40 p.m. at Ashe Stadium.

That was early by comparison to her previous match against Elena Vesnina, a three-setter that ended at 1:45 a.m. on Sunday morning at Ashe. With her first two matches t also ending relatively late, Keys has had her biorhythms disrupted, even though she has reached the quarterfinals of the U.S. Open for the first time.

“Like this is a little insane. Going to bed at 4 a.m. every morning and waking up at 11, it’s not totally natural and normal,” Keys said. “But now I’m afraid to, like, play in the sun because I haven’t done it in two weeks, so . . . I’m fine with playing at night.”

She began strongly against Svitlolina, winning the first set, then started down a slippery slope in the second, down 5-0. She lost it 6-1, and then when Svitlolina’s serve started hiccuping in the third she jumped on the opportunity and closed out the match.

Keys is another comeback story at the Open, having to take the winter off for what turned out to be two surgeries on her right wrist.

“It was a really rough start to my year,” said the 22-year-old American after the match. “This is just amazing. I’m really proud of myself for digging deep and figuring that out tonight.

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Keys plays qualifier Kaia Kanepi in the quarters. It’s Wednesday night at Ashe. Natch.

Major challenge for CoCo

Expect some nuclear tennis when American CoCo Vandeweghe plays No. 1 Karolina Pliskova in a quarterfinal Wednesday at Ashe. That’s what Vandeweghe expects.

“I think it’s definitely the first-strike kind of tennis, I think is the easiest way to simplify it,” she said. “If I’m hitting my shots, I’m definitely doing a lot better job in the score line than she is and vice versa.”

Pliskova beat Vandeweghe on clay this season, but on hard court, it’s a dihard courttory. “[She will be] different match or opposite matches what I had so far this tournament,” Pliskova said, “so it was not many girls which are playing that fast and with that serve and that speed.”

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Big win for LIer Bolton

Elysia Bolton of Cold Spring Harbor scored a significant upset Tuesday when she beat No. 2 seed Marta Kostyuk in the junior girls singles, 7-6 (4), 6-2. Bolton is home schooled and trains full-time at the Billie Jean King National Tennis Center. She is committed to UCLA on a tennis scholarship.

“I think I competed really well, I didn’t get down on myself,” Bolton said. “When I went down I didn’t want to think about losing a set. When I went up I didn’t want to think about winning it, either, just focusing on that point.”