Behind the humility and self-effacing humor was the truth.

Jessica Pegula hadn’t believed. Not in her core did she have down-to-her-toes belief.

But her confidence has grown over the past year. Heck, over the past weeks.

It carried her into Saturday’s U.S. Open final.

And she’s hoping it’ll carry her to her first Grand Slam title. No, make that she’s confident it will.

“If you would’ve told me at the beginning of the year I’d be in the finals of the U.S. Open, I would’ve laughed so hard because that just was where my head was, not thinking that I’d be here,” admitted Pegula, who will face No. 2 Aryna Sabalenka.

Pegula, whose father Terry owns the Buffalo Bills and is worth $7.7 billion, per Forbes, has been a top-20 player since 2021 but came into Flushing 0-6 in Grand Slam quarterfinals.

But the seeds of her breakthrough win over No. 1 Iga Swiatek were sown long before.

When she broke out of her comfort zone, switched coaches, remade her body, improved her movement.

Improved herself.

“Once you start winning a lot of matches and have this belief in yourself that you can win in tough moments and you can beat these girls, it just gives you a lot of confidence going into these weeks,” the 30-year-old Pegula said. “I’ve been able to build off that. Learning through different experiences, winning tournaments, all that stuff just adds up.

“When you do stuff and you try them, you’re working on things and you’re able to use them in tournaments, you get confident and you’re like, ‘I can really get better, I can improve, and I can hit these shots in big moments.’ That just builds, and it can build really quickly. … So, it’s nice. I hope I can be an inspiration to other players that feel they’re stuck or not getting better, that you can always find a way to get better and improve.”

That open mind and hunger to improve included parting ways with longtime coach David Witt after a second-round loss in the Australian Open, and bringing on Mark Knowles and Mark Merklein for February’s San Diego Open.

The confidence boost has been undeniable.

“Yeah, obviously it’s grown a lot,” Knowles told The Post. “She’s been one of the top players for a couple of years, and all players want to make that jump at the Grand Slam level. She’s a very ambitious young lady, and she felt that she had the tools to do it. But it takes another level of belief. And I think she’s there. I think she really gained that confidence this summer.

“Her confidence has really grown, and it’s really shown throughout this tournament. She’s had a difficult draw, navigated a couple of difficult situations. She’s right where she wants to be, and she’s got a belief that she can do it.”

After some early season injuries, Pegula has worked hard on her conditioning, which helped her durability and her movement. That’s shown in her results, a stellar 15-1 since the Paris Olympics.

“Even Jessica after Wimbledon was like, ‘I know I’m playing well.’ So that’s where you need the combination. Then you do need validation of some results,” Knowles said. “And for her to come off the heels of the Olympics and turn around on a different surface and win the tournament in Canada, that’s exactly what she needed. … So when a player gets validation and they feel that they’re playing well, that combination breeds confidence.”

The win in Toronto buoyed Pegula’s confidence, as did making the final in Cincinnati. That’s where her only post-Paris loss came, 3-6, 5-7 at the hands of Saturday’s foe.

“Yeah, it’s a terrific challenge,” Knowles said. “It’s the finals of a Grand Slam. She’s going to let it all hang out. And you want to play [the best]. You have to argue that Sabalenka is probably the top player in the world right now, and that’s who you want to beat. That’s the ultimate challenge. She’s super excited for it.”

And confident about it.