Growing up in New Jersey as “the shortest, skinniest kid” on whatever team he was on, Gerald Marzorati never expected to be playing nationally competitive tennis in his 60s — especially since he started playing at age 55.

His book, “Late to the Ball: Age. Learn. Fight. Love. Play Tennis. Win” (Scribner, out now), describes how he picked up a new athletic endeavor at a relatively late age — and became good enough to enter the National Senior Grass Court Championships six years later, in 2013.

Unlike many of his fellow middle-aged men in Westchester, NY, he wasn’t interested in golf. “It’s a lovely game, but I need to move more,” Marzorati, now 63, tells The Post. “Tennis has everything — speed, endurance, hand-eye coordination.”

The former magazine editor says one of his biggest challenges was getting the technique right in a game with little room for error — being off by a second, or a step, can mess up a shot. Beyond the mental demands of the game are the physical challenges. Though Marzorati came to tennis lean and fit, he’s still upped his workout: In addition to spending several hours a week on the court, he hits the gym for interval training, plyometrics and TRX ropes, as well as once-weekly yoga, on a day he’s not hitting the ball.

Still, tennis can’t stop the clock, and the changes that come with aging aren’t things you can just swat away. “I have bursitis; my knees are arthritic,” Marzorati says. One of the funniest asides in the book is the debate he says rages among senior players: Advil or Aleve? Athletes of all ages know that discomfort is part of life, but you shouldn’t let it stop you.

Marzorati used to suffer from tennis elbow too, but found that acupuncture helped. And since feet tend to splay with age, he sought wider shoes — he prefers Asics.

Marzorati realizes that if it’s not the elbow or the feet, it’ll be something else.

“Obviously, I’m going to diminish at some point,” he says, “but it’s going to feel natural, because that’s what happens when you get old. I just love being out there playing tennis. I love watching it, thinking about it — it’s all an expression of passion.”