Donna Vekic got off the mat so many times in her U.S. Open round of 16 victory, she’s not even sure how she won herself.

But she gutted her way to a come-from-behind 6-7(6), 7-5, 6-3 marathon win Monday over hard-serving German Julia Goerges. In a match that lasted nearly three hours, Vekic staved off match point and fought her way into the first Grand Slam quarterfinal of her career.

“Yeah, I’m happy to win and I’m happy to win saving match points, happy to be first time in the quarterfinals. I’m just happy right now,” the 23rd-seeded Vekic said with an ear-to-ear grin.

“I don’t know how I won this match. She served for it, she had match points. I kept fighting and believing I could win. … I was just trying to get a return in the court. She served amazing today. I felt confident in the rallies and I felt that if I got the ball in, I’d have a good chance.”

Vekic couldn’t have been faulted if her confidence had been flagging. She came into the Open winless in three prior meetings — including last year’s fourth-round Wimbledon loss. And on Monday she almost got served right off the court, with Goerges piling up a mind-bending 21 aces, the most in any women’s singles match since 1998.

But Vekic didn’t fold mentally, digging down and relying on a reservoir of confidence buoyed by some big wins over the past year. Now the Croatian has added her biggest, and a date versus No. 13 Belinda Bencic.

“I’ve always been a player that fights until the end. I’ve always been a fighter,” Vekic said. “But since last year, since I had a few top-10 wins, I really got a lot of confidence from that. I managed to build on that.”

Goerges’ serve had kept Vekic on her heels through the second set, and she had a chance to clinch the match before a pivotal game where she committed not one, not two, but three double-faults, and tossed in a time violation for good measure. It proved the turning point.

After taking the second set, Vekic seized on another opportunity in the third. She broke thanks to a double fault by Goerges, going up 5-3. Then Vekic showed some fortitude in staving off a couple break points and closing out the win.

“Yeah, definitely I changed a lot mentally this year. I improved a lot in believing I could win at any given moment, even when I’m match points down,” said Vekic, citing wins like her Toronto comeback over Madison Keys.

“But, yeah, when the matches are this close, even if you’re match points down, you have to believe you can win. I was really going out there, when she was serving for 5-4, I was really thinking I could win this game and try to turn the match around.”