One Grand Slam title short of tying Margaret Court’s all-time record, Serena Williams has reached three finals since becoming a mother — and lost them all. But she and her camp vow Saturday will be different.

They say she got to those finals on winning DNA, not because she was actually ready to come back and win. But now in her best mental and physical shape since childbirth, Saturday’s U.S. Open final against Bianca Andreescu should be Williams’ to lose.

“That’s the best she’s been since she became a mother. By far,” Williams’ coach, Patrick Mouratoglou, told The Post.

Mouratoglou is so confident that — after proclaiming he would give on-court coaching again if she struggled — he said, “It’s not going to happen, so I won’t have to.”

If there’s a fountain of youth, it might be in Williams’ living room.

Williams, who will turn 38 later this month, is trying to become the oldest player — man or woman — to win a Grand Slam in the Open Era. Her 19-year-old foe wasn’t even born when Williams claimed her first title, in Flushing in 1999.

At least so far, motherhood has been a tougher foe than Father Time in Williams’ quest for No. 24.

“It’s amazing to come back with a baby and win, because it’s hard. My day off isn’t a day off,” Williams said. “I’m a full-time mom first, foremost. That means the most to me.”

Only three women ever have won majors in the Open Era after becoming mothers: Court, Evonne Goolagong and Kim Clijsters.

“Her body transformed to become a mother. You don’t come back to your previous body — which is the body of a professional top athlete — overnight. It takes time,” said Mouratoglou, adding, “Her movement now is better than I’ve ever seen since she became a mother.”

Health is the key for Williams. Playing with an injured shoulder in the 2016 Rio de Janeiro Olympics, she lost to Elina Svitolina; she came back and routed the Ukrainian in the Open semifinals Thursday. Her back forced her to retire during a match last month in Toronto against Andreescu, but Saturday affords her a chance at payback — and history.

“There’s so many different emotions in finals,” Williams said. “It just brings out so many highs and lows, nerves and expectations. It’s a lot.”

Williams has been stuck on 23 Grand Slam wins since giving birth to her daughter, Olympia, two years ago. She has reached three finals since — Wimbledon (twice) and last year’s U.S. Open — but dropped them all.

“Wimbledon? Oh, that doesn’t count. God, I had like an 8-month-old,” Williams said.

“It’s difficult for us to realize, because she was in three finals, so you feel she’s ready,” Mouratoglou said. “But she was in the three finals because she’s the best competitor of all-time, not because she was ready.”

She will have to be Saturday. Andreescu is 43-4 this year, including her Toronto win over Williams.

“She really knows how to mix up the game and play different shots in different ways,” Williams said. “Above all, I just like her as a person. She’s amazing.”

“I remember watching her when I was about 10,” Andreescu said. “I think she’s fighting for her 24th on Saturday. I’m sure she’s going to bring her ‘A’ game. I’m going to try to bring my ‘A’ game, too. … May the best player win.”

Andreescu’s coach, Sylvain Bruneau, called the young Canadian a “warrior” and a “street fighter,” while Mouratoglou added he picked her to reach the final and expects her to be No. 1 soon.

“Not too soon,” he added, “but in the future.”

The present is supposed to be Serena.