American qualifier Taylor Townsend stunned the tennis world Thursday night with the biggest upset of this year’s U.S. Open — and the biggest breakthrough of her career.

Townsend rallied to knock off fourth-ranked Wimbledon champion Simona Halep 2-6, 6-3, 7-6 in a match that had the Arthur Ashe Stadium crowd cheering — and the young Chicagoan in tears.

“[Before], I played not to lose. Today, I played to win. I just said F- it,” Townsend said in an on-court TV interview. “I don’t think I can really say what was going on in my head because of TV. But I told myself this is an opportunity, a chance to go for it. You have nothing to lose.”

Townsend had done enough losing, with one of the more galling defeats coming when she squandered match point on Kiki Bertens in the second round at Wimbledon. On Thursday, she squandered a pair of match points, double-faulting as the pressure mounted.

But Townsend — ranked No. 116 — didn’t fold, staving off a Halep match point herself to eventually pull out her first-ever win over a top-10 foe. She’d been 0-for-10 before beating the Romanian.

“It’s been a long road. A lot of haters, a lot of people who weren’t sure. I’ve heard it for a really long time that I was never going to make it, that I wasn’t going to be able to break through or this or do that,” said Townsend, who once had her support from the USTA shockingly pulled due to her weight, despite being the top-ranked junior in the world.

“For me, especially over this last year, I’ve really embraced that, thriving in these kind of conditions, being able to prove people wrong. I’ve always been that way, but I suppressed that side of me for a long time. I’ve just embraced it and it’s like, ‘This is the person I am.’ I love it. I thrive in these kind of conditions and it showed [here].”

Seemingly more comfortable in her own skin, Townsend took the game to the two-time Grand Slam winner. She went all-in on her chip-and-charge game, coming to the net a staggering 106 times and winning 64. She dared Halep to lob her, riding her aggression, net play and booming forehand to victory.

“I feel like a lot of the times when I was playing her, I played not to lose. A player like that, she’s just way too solid, has had so much experience, that’s not going to fly,” Townsend said. “I was just like, what do you have to lose? I’m just going to go for it, I’m going to do what I do best where I’m the most comfortable, which is at the net.”

Townsend tried to serve out the match up 5-4, but got broken. And after Halep held to go up 6-5, Townsend forced a third-set tiebreak. And once she got there, she won a nervy 7-4 tiebreak for her fourth straight come-from-behind win, including qualifying.

“This means a lot. It’s been a long journey,” said Townsend, covering her face with yellow-painted nails and crying. “It gives me so much confidence to know I can do it.”