Karolina Muchova admitted she had been nervous at the start of Monday’s fourth-round match at the U.S. Open. Turns out, the worry was wasted.

The Czech — finally fully healthy after wrist surgery — drilled No. 5 seed Jasmine Paolini 6-3, 6-3 in just over an hour to reach the quarterfinals.

Paolini had made the finals at both Wimbledon and the French Open, making this an upset. But it was an upset on paper, not in reality.

Last year Muchova had reached the semis here and the finals at the French Open, her ranking reaching a career-high No. 8.

But wrist surgery cost her 10 months, admitting in an on-court TV interview it was the “worst and most serious injury” in a career with plenty.

Muchova finally came back at Eastbourne in June, with the U.S. Open just her sixth tournament since. Ranked just No. 52, she hasn’t dropped a single set in Flushing.

On Monday her serve-and-volley game was strong, and she won 11 of her 14 net points — 60 out of 79 so far in the tourney.

“I’m very happy. I’m really very happy,” Muchova said. “I was today nervous, and I felt as well in the first games that I was going backwards with the shots. It doesn’t happen much to me, so I was a little bit surprised about that.

“But, yeah, it’s important for me. I knew it’s important match. I just wanted to do good. So maybe it’s all of these feelings about that. But, yeah, now after [the] match I’m just very happy to be in quarters here.”

Now Muchova — who hit 17 winners — goes for a second straight U.S. Open semi. She’ll face Beatriz Haddad Maia, against whom she’s a perfect 3-0.

“Against Bia, we played I think last year I guess as well. Maybe we played in Cincinnati, very tough battle, three sets. She’s a big fighter, lefty,” Muchova said.

The 22nd-seeded Brazilian Haddad Maia beat former World No. 1 Caroline Wozniacki 6-2, 3-6, 6-3.

“For sure it will be a tough battle. Last time we played it was an amazing match from both of us. … I’m very happy to have this opportunity to play big players on a big stage. This is what we work for,” Maia said of Muchova before tearing up as the crowd chanted, “Ole, ole ole ole, Bia, Bia!”

“I try to hold the emotions until the end of the match, but now I couldn’t. It’s very special for me.”

A year ago she had never been past the second round in a major. Now Haddad Maia will play for a berth in the semis after beating the former Australian Open champion.

“Obviously as an athlete, matches like this, you’re disappointed to lose and not be through. I think it was a tight one. I didn’t start off my best. I was finding it hard to find a rhythm. The lefty,” Wozniacki said. “I came in there trying to win the match and wanting to win and believing that I was going to win.”

No. 25 Jack Draper of Great Britain drilled Czech Tomas Machac 6-3, 6-1, 6-2 in the fourth round.