GOATs are supposed to be stubborn. And Wednesday night, Serena Williams was supreme, the greatest of all time refusing to let Anett Kontaveit end her career.

Williams has said she’s “evolving” away from playing — she won’t say retire — but she’s clearly not done writing her story. Unseeded but undaunted, she fought off a tough effort from second-seeded Kontaveit, for a 7-6, 2-6, 6-2 second-round win at an electric Arthur Ashe Stadium that staved off her career curtain call.

“The experience was really good. You can only have this experience once in a lifetime, for sure,” said Williams. “I’ve been practicing really well, but it hasn’t been coming together in matches. But now it’s kind of coming together, I guess. I mean, I had to bring it together [Wednesday]. It worked out.”

It worked out with a gut-check third set, defying her age by moving better than she has all year. She clinched a turn-back-the-clock performance with her typical victory twirl, a fist pump and a wave to the crowd, along with a long, lingering nostalgic look toward her player’s box.

That box included not just sister Venus, mother Oracene and husband Alexis Ohanian, but also Tiger Woods — without whose help and support, she insists she might not still be playing today, still eyeing her milestone 24th Grand Slam title.

“Yeah, he’s one of the reasons I’m here, one of the main reasons I’m still playing,” Williams said. “We talked a lot. He was really trying to get me motivated. … We were like, ‘OK, we can do this together.’ It was good, because I didn’t know what I wanted to do. I was just lost, so many questions. When you can rely on someone like that — my goodness, he’s Tiger Woods — it was really helpful to get clarity.”

Clad in the same glittery crystal-encrusted top and diamond-accented sneakers — with Mama on the left, and Queen on the right — Williams moved better than she had Monday. And better than expected for a mother set to turn 41 later in September.

She took the first set not with her vaunted offense, but her defense.

Williams fought off a break point to avoid going down 3-1 early. She rode that defense to earn three break points against Kontaveit in a 12-minute, 20-point game at 3-all. And despite not being able to close out any of them, Williams kept coming and applying pressure.

Finally, after failing to convert her first five break points, Williams finally broke through against Kontaveit to go up 5-4 and serve for the set.

Catch all the action from Serena Williams’ final US Open tournament before her retirement. The New York Post has live coverage, updates and analysis as she faces Anett Kontaveit.

Her second-seeded foe turned the tables on her, with Williams going down love-40 after a 20-shot rally. She couldn’t serve her way out of trouble.

But she did hold to stay in the set, and gutted out the tiebreaker. Up 5-4 with two serve points coming, her ace and muted double fist pump finally put the set to rest after 63 minutes.

Kontaveit — who has struggled in big moments against big players — had never gotten past a quarterfinal in any Grand Slam event. But she got a quick break at love, then broke again to go up 3-0, serving her way to a 6-2 second.

One had to wonder where Williams’ endurance would be after having just a half-dozen matches all year. Apparently it was just fine.

Williams earned three break points before converting to go up 2-0. A swinging forehand volley for a winner gave Williams a 5-2 lead. She earned triple match point on the Estonian’s serve, and clinched it with a backhand winner.

“I feel like everything is a bonus,” Williams said. “I’ve had a big red ‘X’ on my back since I won the U.S. Open in ’99. It’s been there my entire career, because I won my first Grand Slam early in my career. But here it’s different: I feel like I’ve already won, figuratively, mentally.

“It’s just pretty awesome the things that I’ve done. I never accept that; I never think about it. Yeah, so [Wednesday] I was just like, ‘Serena, you’ve already won. Just play. Be Serena. You’re better than this.’ That’s what I was able to do.”