Kayla Harrison enters an ordinary door on a gray street on the outskirts of the Garment District. She walks past the doorman, who doesn’t look up from his newspaper, and goes down the stairs to arrive at Renzo Gracie’s jiu-jitsu academy.

There is no entourage, no handlers, no TMZ, just her and a bunch of sweaty fighters who pay her no mind.

You wouldn’t know she is the most successful American judo Olympian in history. That she is far more accomplished in her sport than Ronda Rousey ever was.

And she would prefer if the inevitable comparisons to Rousey stop right there, even as she seems set to follow in Rousey’s path.

“In terms of what I would learn from her …” Harrison told The Post, her voice trailing off a bit as she thought about her former teammate. “I don’t know. … It’s such a different world and her rise to fame was just like almost overnight, and I think that’s a lot of pressure to deal with.

“For me, it’s about staying true to my roots. I don’t want to have any yes-men, is what I like to call them. I want to have people around me who are going to be honest with me, who are going to be like, ‘Look, you need to get your butt out of bed and start training because you’re slacking.’ I think that’s what I learned from her.”

Harrison will take everything she has learned from Rousey with her when she starts her own MMA career. The 26-year-old Ohio native recently signed a four-fight contract with the World Series of Fighting and also enlisted as a brand ambassador and on-air commentator for the company. Though her debut date inside the cage is to be determined, she will get her first chance behind the microphone at the WSOF’s huge card on New Year’s Eve at Madison Square Garden.

In choosing the smaller promotion, Harrison turned down an offer from the UFC, according to the Boston Herald, to slowly and steadily build herself into the best MMA fighter in the world.

She says she is going to achieve her goal while staying “classy.” But no matter what she does, the comparisons to Rousey are unlikely to dissipate.

The two women lived together, trained together, competed together and became world-class judokas together. Then they chose different paths that ultimately led to funhouse-mirror versions of similar glory.

Rousey’s path led her to MMA stardom and eventually the UFC. She took her judo hip throws and armbars into the Octagon, became a juggernaut of Mike Tyson-like destruction and earned a UFC title belt.

Blazing an unprecedented trail, Rousey picked up the nickname “Rowdy” — after WWF legend “Rowdy” Roddy Piper gave her his blessing — posed in body paint for the Sports Illustrated Swimsuit Issue, and starred in blockbuster movies, including “Furious 7,” alongside the likes of Dwayne “The Rock” Johnson.

Harrison watched Rousey from afar while sticking to her judo roots. Unlike Rousey, who has been open about her disdain for certain parts of the United States national team experience, Harrison found her calling through judo and her coach Jimmy Pedro, a two-time Olympic bronze medalist. Pedro was there for Harrison at the darkest point in her life, and he still is by her side today.

For three years, starting at age 13, Harrison was the victim of sexual abuse at the hands of her then-coach Daniel Doyle. Harrison provided a witness statement against him, and in November 2007, he was sentenced to 10 years in prison.

In the traumatic aftermath of the abuse, Harrison’s mother reached out to Pedro, who took in Harrison at his Boston-area gym and made her a protege. He became more than just Harrison’s judo coach, he became family. Together, over several years, they forged Harrison into a judo master and champion for sexual abuse victims. Harrison started the Fearless Foundation, which describes its mission as to “shine a light on the darkness that is child sexual abuse and to enrich the lives of survivors through education and sport.”

Back in the judo world, Harrison’s hard work finally paid off after Rousey won a bronze medal at the 2008 Olympics in Beijing. Harrison stepped into the top spot on the American judo team and surpassed everything her former teammate achieved. She surpassed anything any American had achieved.

At the 2012 Olympics, Harrison became the first American, man or woman, to win a gold medal in judo. This year in Rio, she defended her title in dominant fashion, defeating all four of her opponents with the equivalent of a knockout — an ippon, a bout-ending maneuver that gives a maximum score.

Given her pedigree, it only was natural for people to wonder whether Harrison would get straight into the cage.

Harrison passed.

“You know, after a [judo] match you shake the person’s hand and you give them a hug and you bow to them to show respect, and in MMA it’s not like that,” Harrison said shortly after she won in Brazil.

“I don’t know if I’m cut out for a world where you get fights based on how pretty you are and how much you talk, not necessarily what’s your worth in the ring. For me, right now, it’s just going to focus on being two-time Olympic champion, enjoy the moment and live in the moment. Never say never, but right now the answer is no.”

It didn’t stay that way for long. Harrison has started informal training in boxing, jiu-jitsu and wrestling, and in January she is planning to take her MMA training to the next level with a full-on training camp directed by Pedro. Though she wouldn’t put an exact date on it, she said she is planning to enter the World Series of Fighting cage in late 2017.

“MMA is changing really rapidly,” Harrison said. “I was very concerned about doing MMA just because of the entertainment aspect. It’s very different from the Olympic field of play.

“In MMA there’s a lot of trash talking. There’s sort of two routes that you can go. There’s one route where it is very similar to WWE, where it’s this smack-down, throw-down, he-said-she-said, flip-off-the-camera-type attitude. Or there’s the sportsman attitude and showing respect. That’s what martial arts is rooted in, that’s the culture of martial arts. It’s respect, discipline, integrity and self-control.”

Whatever the future brings, Harrison is determined to move forward on her own terms.

“I think the real difference is that I’m going to do it the Kayla way,” she said. “I’m not trying to be the next Ronda Rousey. I want to be the first Kayla Harrison.”