At 35, most tennis stars are considered over the hill. Serena Williams? She’s looking for bigger mountains to scale.

The past few months have been blockbuster for the champ. In January, she set a world tennis record by winning a 23rd career Grand Slam title. The month before, she got engaged to 33-year-old tech millionaire Alexis Ohanian, a Reddit co-founder, in Rome. To top it off, this week, Page Six reported that Serena is slated to grace the cover of Sports Illustrated’s Swimsuit Issue, out Monday.

It’s a welcome turn of events for Serena, who, over the years, has experienced hardships: a life-threatening blood clot in her lung, the tragic murder of her half sister and personal assistant Yetunde Price and racist jeers from onlookers during matches, including one incident at Indian Wells that caused her to boycott the California tournament for 14 years.

“I think it is the happiest she’s been,” Williams’ mother, Oracene Price, told The Post of her daughter.

“She’s pretty tough on herself and she’s such a competitor, sometimes I don’t think she’ll ever be satisfied.”

On the court, maybe not. But when it comes to romance, Serena seems pretty content since becoming engaged to Ohanian, whose social news aggregation and discussion board, Reddit, is the seventh most visited Web site in the US, according to web-analytics firm Alexa.

“I think they’re a great match,” said Robbye Poole, Serena’s hitting partner of two years. “Alexis is very supportive, caring, super laid-back. So he knows how to deal with the times when she’s under a lot of stress. They balance each other well.”

“He’s good for her because she’s one of these people with a Type-A personality, and he’s more calm and serene,” added Price.

The pair first met at a lunch in Rome in May 2015 (he would later pop the question at the same restaurant). Before the tech mogul, Serena was involved in a series of high-profile romances, such as musicians Common and Drake, basketball star Amar’e Stoudemire and movie director Brett Ratner.

“They came and they went,” admitted Price of Serena’s previous suitors. “It’s difficult for women in [Serena’s] position to find someone that really cares for them.”

But those close to Serena say Ohanian is different.

She wants to be the best player possible. Every time she steps on the court, her goal is to win

Her mom recalls how the 6-foot-5 entrepreneur, who is based in San Francisco, hopped on a flight to Southern California to ask her for permission to wed Serena, who is worth an estimated $150 million.

“I think he was very nervous,” Price admitted. “He was going to do it anyway, but he was following the proper etiquette.”

And while news of the power couple’s glitzy engagement and post-proposal trip — replete with helicopter jaunts — to New Zealand took the Internet by storm in December, Serena’s mother said it’s the simpler things that show Ohanian’s devotion.

“There was this shirt [Serena] liked but couldn’t find it, a black-and-red check shirt,” she said. “And he went online and found it for her. To me, that’s romantic because you’re listening to the person you’re caring about say something and you’re doing something about it.”

Serena may have found true love, but she’s not letting it distract from her day job.

Price said that with age, her daughter has learned to listen to her body — eating better, training more wisely and playing fewer matches.

“She’s at the top of her game now because she’s smarter and has more experience,” said Price.

“She wants to be the best player possible,” added Poole. “Every time she steps on the court, her goal is to win.”

Which makes losing all the harder for the superstar to cope with.

In their younger years, when Serena and her sister Venus, now 36, lost a game, they would simply run off the court, according to Price.

“Serena loves competing and loves the trophy. So if she and Venus lost, they would just leave their [prize] money there because, to them, all they cared about was bringing home the trophy and accomplishing the goal,” said Price.

“The accountant found a lot of checks that they had left behind. He had to change the way they got paid,” she said, with a laugh.

Sascha Bajin, Serena’s hitting partner of eight years prior to Poole, said the tennis star puts pressure on herself to win at everything — whether it be a match or a game of Uno.

“She hates losing,” said Bajin. “I think that she takes losing harder than anything else in her life because she puts so much pressure on herself and the media puts so much pressure on her. Even for just losing a set.”

As Serena told the New York Times Magazine in a 2015 interview: “I play for me, but I always play and represent something much greater than me. I embrace that. I love that. I want that.”

Serena has known pressure since a young age.

Even before she was born, her father, Richard Williams, famously watched the TV in awe as Romanian tennis player Virginia Ruzici walked home with a $40,000 check after winning a tournament.

Amazed that a female athlete could reel in that kind of money, Richard, who had no tennis experience himself, decided he would create the sport’s next female superstar.

At first, he tried training his then-wife, Price (they divorced in 2002). Then, his three stepdaughters, Price’s daughters from her first marriage. But it was when Venus and Serena were born that Richard knew he had found his moneymakers.

Richard and Price taught their girls the ins and outs of tennis, which was mainly known as a white, upper-crust game in gang-ridden Compton, Los Angeles. He wrote in his 2014 memoir that he went so far as to bus in school kids to hurl insults like the N-word at his two young daughters while they played in order to toughen them up.

“If someone says something negative, you should never come back with something negative to say,” Price said of the family mentality. “It will make you a better person and stronger, and will help you deal on and off the court.”

That mentality was tested in 2001 at the Indian Wells tournament in Indian Wells, Calif., when Serena was booed and jeered by the well-heeled crowd. Richard told media outlets at the time that he heard one guy sneer, “I wish it was ’75; we’d skin you alive.”

Bajin said that during his near-decade training with Serena, “She hasn’t been treated well.”

“Incidents happened a few times and I would ask her if I should step in and do something — and she never said yes,” said Bajin. “She said, ‘Let it go, it’s not worth it.’ It just shows how much bigger she is.”

Two years after Indian Wells, Serena’s personal assistant and half sister Yetunde was murdered in a drive-by shooting in Compton.

Serena, who had just undergone surgery to repair a torn knee tendon, plunged into a deep depression; her career took a turn for the worse over the next two years, with her ranking plummeting from No. 3 to 139th in the world.

She decided to take a breather from tennis. “She got a chance to rest and get a little bit more reflection,” her mother said. “She needed a mental break.”

When Serena returned in 2007 for the Grand Slam in Australia, ranked No. 81, she was ridiculed by the press for the extra weight she had gained.

“One of them called me a ‘fat cow.’ I was like, ‘Is this for real?’ It wasn’t easy,” Williams told the Times UK in 2009. In a remarkable comeback, she won the tournament.

Serena’s’ career hit another bump in 2010, when she received stitches on both feet after stepping on glass at a restaurant in Munich. Months later, she underwent emergency treatment for a blood clot in her lung, which sidelined the athlete for 12 months.

And yet, when she returned, Serena was better than ever. At age 31, she became the oldest tennis player to be ranked No. 1. The next year, in 2014, she won her third consecutive US Open title. In July 2016, she won Wimbledon for the seventh time, the second most wins ever (Martina Navratilova has nine titles).

And while there are still trials for Serena to overcome — after her win at the 2015 French Open, brutally cruel remarks about her looking like a gorilla and a man were rampant on Twitter — she’s not relinquishing that No. 1 ranking any time soon.

“Everything she’s been through, that just makes her stronger,” said Poole. “It helps her get through the tough times when she’s playing because she realizes she’s been through worse.”

Right now, Serena — who splits her time between her mansion in Palm Beach Gardens, Fla. (one of three Florida homes she owns), and her Paris apartment — is enjoying life with her betrothed and spending time with friends like La La Anthony and fellow tennis player Caroline Wozniacki.

When she’s not training or competing, Serena’s working on her eponymous HSN clothing line or hosting her annual Williams Invitational in Florida, where friends take part in a massive dance competition, dodgeball tournaments and, yes, a round robin of tennis.

(Price said Ohanian has yet to make the cut for one of the dance teams: “If he can’t dance and compete, he won’t be invited into the competition. Serena likes to win,” she added, with a laugh.)

Price believes that children are definitely in her daughter’s future: “That’s why she has so many dogs.”

With marriage and family on the horizon, industry insiders are wondering whether Serena’s retirement will follow closely behind.

Knowing her, they’ll likely be left wondering.

“I remember when we were at the Olympics in London, she said, ‘For sure, I would love to win the Olympics and retire in Rio.’ Then Rio came around, and she’s still No. 1 in the world,” said Bajin. “You just don’t see an end to her career. She’s working against time.”