For the third straight match, Serena Williams needed to go the distance.

And for the third straight time, Williams showed the fitness and fight at age 38 to rally to a three-set victory — this time 4-6, 6-3, 6-2 over Bulgarian Tsvetana Pironkova — and charge into Thursday’s U.S. Open semifinals. She is two wins away from tying Margaret Court with 24 Grand Slam titles.

The unseeded and tricky Pironkova put a mighty scare into Williams by disrupting her rhythm with a rare slice forehand out of the paddleball courts of nearby Rockaway Beach, and followed them up with precise two-handed backhands to the open court.

“It’s not how you start; it’s how you finish, right?’’ Williams said. “Some of these matches, you’re playing Serena, so it’s, like, close your eyes and hit as hard as you can and it doesn’t matter. So I kind of have to adjust to that.”

Williams, who banged out 20 aces, next faces Victoria Azarenka, who defeated Elise Mertens Wednesday night, 6-1, 6-0. Eventually red-hot Naomi Osaka may stand in her way in a rematch of the controversial 2019 finals.

Responding to Pironkova’s slices, Williams did her unorthodoxy one better during a key game in the second set.

Williams switched to her left hand — paddleball style — to return a monster serve from her backhand side, showing her reflexes and athleticism. The ball weakly cleared the net, but an off-balance Pironkova whipped it long.

That set up a break point for Williams and she outlasted the Bulgarian baseliner in a 24-shot rally ending with a Pironkova drop shot hitting the top of the net. Pironkova fell onto her back in exhaustion.

“That was intense,’’ Williams said. “At that point I was fighting so hard. Actually been hitting a little lefty in my practice, but not on purpose. I noticed the other day, I hit a lefty shot and I was just in practice being super intense, and it came through in the match. I was just trying to stay, this is the quarterfinals of a Grand Slam, and I was just trying to do everything I can whether righty or lefty.”

The Compton, Calif., product then broke a tiring Pironkova on her first service game of the third set amid a five-game winning streak. Williams’ stamina showed in full force as she stares down turning 39 later this month.

“Age is really how you feel mentally and how your body is doing and how you physically are able to keep up,’’ Williams said. “If you think of it as just a number, then that’s all it is.”

Perhaps this should’ve been much tidier for Williams. Before the Open, the 32-year-old Pironkova had played her last tour match over three years ago at Wimbledon in 2017. During her time away from the tour, she became a mother and didn’t want to return so quickly because of the baby.

“I could barely win a match when I came back,’’ Williams said, saluting her opponent. “She’s incredible. It just shows me how tough mothers are. If you can birth a baby, you can do anything.’’

Williams became a mother three years ago as well, but she’ll be the one rolling to the semifinals.

The contrasting styles were entertaining to watch: Williams offered sheer power, while Pironkova would change up the tempo before crushing a well-angled backhand.

“Definitely the first set I think I was in control of the match,’’ Pironkova said. “I was doing all kinds of shots, and everything went in my way. I was expecting it’s not going to last forever. She’s going to try different things, put more power in her shots, and that’s what she did.”