Jack Kennedy practices with Botic van de Zandschulp during the U.S....

Jack Kennedy practices with Botic van de Zandschulp during the U.S. Open at the USTA Billie Jean King National Tennis Center on Friday. Credit: Errol Anderson

Mastering something inevitably involves repetition, and that must have felt especially true for Huntington’s Jack Kennedy when he stepped into the Billie Jean King National Tennis Center as an athlete instead of a spectator.

It took thousands of hours of work to get here — no small chunk of your life when you’re only 16 — and on Monday morning, it took just an hour and 10 minutes for it to end. But that wasn’t the take-away from his 6-4, 6-1 loss to No. 4 Mees Rottgering, the world’s fifth-ranked juniors player, in the first round of the boys’ singles tournament at the U.S. Open.

The unseeded Kennedy may have lost to a formidable opponent, but here, on one of the biggest stages of his life, he was able to get some important reps in. Maybe just not the usual kind.

“With the big moment and the pressure, I think it’s a different match, a different kind of sport,” Kennedy said. “Experience is a big thing. [Rottgering] is a year, maybe two years ahead of me and he’s played here before . . . He’s used to the moment a little bit more than I am. It’s my first time being here, my third Grand Slam. I’m a little younger, but I think with time, I’ll be able to get used to this kind of moment.”

There are whirlwinds, and there’s what Kennedy experienced during the past week. The No. 32-ranked junior in the world earned a spot in the Open qualifiers after reaching the finals of the United States Tennis Association Boys 18s National Championships last month. Kennedy put on a good show in his U.S. Open qualifying match before falling to Germany’s Maximilian Marterer, ranked 100th in the world, 6-3, 6-4.

When he learned he’d be facing Rottgering, he looked for another lefthander to practice against. He ended up sparring with Botic van de Zandschulp (you know, the guy who ousted Carlos Alcaraz from the U.S. Open last week).

There, friends and family filed into Arthur Ashe, chanting his name before he was whisked away to do an interview with ESPN.

“I’m sure there’s disappointment there for him, but we’re just super-proud,” said his dad, Bryan Kennedy. “It was just surreal. I watch this every year and these [former] players, these pros [on ESPN], they’re talking about my kid and cutting to the interview he did and talking about how well he did, and I was so proud of him, how he handled it all . . .

“Him playing in the qualifying match, having a crowd cheering for him and him soaking it in and just seeing the smile on his face was probably the best part. If I had to boil it down, every time he smiled made me happy, because you knew he was having fun.”

Jack also has just begun. In addition to beginning his college search, he has a packed slate ahead of him. He’ll be training for the next month before a flurry of tournaments: two in Texas, the Junior Davis Cup, an ITF tournament in Turkey, the Orange Bowl in Florida, and, he hopes, the Australian Open and a return to Flushing Meadows this time next year.

“An experience like this, he can learn from it and it’ll make him hungry to get back here,” Bryan said. “At his age, to be in a tournament like this I think is a huge accomplishment . . . We’ve grown up coming here. For us to be sitting up on the top of Arthur Ashe to him practicing on that court [means so much].”

The draw against Rottgering was tough, sure. And though Kennedy was still shaking off the loss, he was able to call his time here “a dream come true” even minutes after walking off the court.

He had about 20 family members and friends there — mind you, ticket prices at this year’s Open are not cheap — and even had support from a tennis fan he met on the plane back from Indian Wells, who looked him up and stopped by to watch his match.

“It’s really special,” Jack said. “To play pretty much in my backyard — you know, I’ve been coming to this tournament since I was 6 years old, so it’s really a special moment to walk out onto that court and hit a couple balls.”

It must’ve been. And now he has a few more reps in. You know, for next time.