Brad Gilbert likes tennis so much that he has trouble sleeping. “I get up at three,” Gilbert said the other day, at the Malibu Racquet Club, outside L.A., while some middle-aged hackers played doubles. “I love waking up, thinking of all the details. My entire adult life, I’m just a tennis guy. I’ve never had a moment where I don’t like doing and being a part of the tennis.”

Life, so far, in the tennis: player (top five), author (“Winning Ugly”), commentator (ESPN), actor (“ ‘Red Oaks’ on Amazon. I was Dr. Feinberg—the club champion!”), real coach to real players (Agassi, Roddick, Murray, Coco Gauff), and, recently, real coach to fake players (Zendaya, Josh O’Connor, and Mike Faist, for “Challengers”).

“I did Z before I started coaching Coco,” he said. Zendaya rooted for Coco from afar as she won last year’s U.S. Open, but they’ve yet to meet. “Coco said she sent her the biggest bouquet of flowers that she’d ever gotten in her life. Z came to the finals of Indian Wells, but Coco lost in the semis. She was so gutted.”

Gilbert, who is sixty-two, was wearing a bucket hat atop a bald head, with a neck gaiter and Nike sweats. His phone buzzed with texts from Gauff’s agent. He’d planned to be with her for the Madrid Open, but he’d been waylaid by dental work.

Gilbert got the movie gig through his daughter, Julian—a reverse nepo-baby situation. Julian was working for the producer Amy Pascal, who had the “Challengers” script. “She happened to tell Amy, ‘My dad’s, like, the tennis guy,’ ” he said.

Movie coaching wasn’t like professional coaching. “At first, they would practice with just the butt end, no head, and no ball,” Gilbert said. But he used the same incentive strategy he does with the pros. “I always had Jolly Ranchers,” he said. “When I’d give them to Coco, she’d be, like, ‘Stop!’ But then, after she won the Open, she got me probably ten thousand Ranchers. The biggest fuckin’ box you have ever seen. And a bunch of flavors that I’d never even seen before. I have a couple of flavors that are bad luck, peach and raspberry.” Why? “It’s just bad luck.” He added, “My grandfather drove a cab in San Francisco for fifty years, and he always had Jolly Ranchers in his cab. Literally, my entire career, I always had a Jolly Rancher in my mouth. Sometimes I do on ESPN. They get pissed. ‘B.G., spit out the fuckin’ Rancher!’ Bad habit. That’s why my teeth are fucked up.”

Gilbert’s wife, Kim, also consulted on the film. She sent the actors tennis tape to study. O’Connor’s character was modelled on a poor man’s Nick Kyrgios, and Zendaya’s on tall bruisers like Maria Sharapova, Venus Williams, and Aryna Sabalenka. Faist’s character was an elegant player. “Like a Fed or Sampras, with a one-handed backhand,” Gilbert said. “Mike was a high-school player with a two-handed backhand. He was so pissed, like, ‘Fuck, I don’t wanna hit this one-handed backhand!’ I was, like, ‘Dude, I didn’t write it!’ ” The three worked together, with Gilbert, every day, for six weeks straight. “The last day of our practicing routine, Z got a pin made with me on it, with hair, from, like, the nineties,” Gilbert said. “She gave it to everybody. It kind of choked me up.”

Gilbert had some experience coaching famous non-players. He trained Robin Williams for a charity doubles match with Andre Agassi against Pete Sampras and Billy Crystal. “Billy was giving him shit that he was taking the tennis seriously,” Gilbert said. A friend of Gilbert’s, the basketball player Chris Mullin, once showed up at his house with Michael Jordan. “They knocked on the door, and it was, like, ‘Oh, shit!’ ” Gilbert said. It was Jordan’s first time playing tennis; he tried to wager on it.

Gilbert grabbed a ball and practiced some serve tosses. Luca Guadagnino, the director, had Gilbert choreograph all the tennis points for the movie. Gilbert also lined up a former player for a bit part as Faist’s coach in the film, Karl, but the guy cancelled at the last minute. “The A2”—the second assistant director—“he was, like, ‘B.G., you’re gonna be fucking Karl,’ ” he recalled. “Next thing you know, they put the wig on me, put the mustache on me, put these clothes on—there’s Coach Karl.” He appears for a few seconds, yelling, in a heavy German accent, “More aggressif! ”

Gilbert’s phone buzzed again. It was Gauff’s dad, letting him know that the team had found Gilbert a 7 a.m. flight to Nice the next day. It was going to be a busy three months—Italian Open, French Open, Wimbledon, coaching and commentating. But he’d find time to play. “I go hit on the wall at least two, three days a week at 6 a.m.,” he said. “I hate coming back and missing balls. Let’s say I’m not gonna play—if I’m in my room, I just swing my racket. Then you don’t get blisters.” ♦