“I’m sweating,” Caleb McLaughlin said, sounding surprised. “I never sweat. I usually get moist. A little damp. I’m sweating!”

This was on a sultry Sunday morning at Midtown Tennis in the Chelsea neighborhood of Manhattan. Mr. McLaughlin, a star of the Netflix drama “Stranger Things,” had spent the last 40 minutes running drills with Nikola Ristic, a brisk and lively pro.

“I’ll make you sweat,” Mr. Ristic had promised, in his Serbian-accented way. “No-mercy style!”

Mr. McLaughlin, 20, grew up in Carmel, N.Y., about 50 miles north of New York City, dancing, singing and acting in community theater. But he was also a sporty kid, playing football, basketball, baseball and tennis.

“I found tennis very therapeutic,” Mr. McLaughlin said. “There’s not a lot going on. There’s only you and one other opponent. And I love the sound of the ball.” He acknowledged that he loved the smell of a tennis court, too, though not perhaps at Midtown Tennis, which felt airless and mildewy that morning.

Mr. McLaughlin had arrived bouncy and fresh faced, just before 8 a.m., a security detail in tow, dressed in a pale green Nike warm-up jacket and black joggers and shoes. He had attended the “Stranger Things” season premiere at the Netflix headquarters in Brooklyn the night before, but the late bedtime didn’t seem to have caught up with him.

He hadn’t been back on a court since “Stranger Things” began filming, about six years ago when he turned 14, and was curious if his skills had atrophied.

“It comes back really quick,” Mr. Ristic said reassuringly, handing him a blue racket. “You have nothing to worry about.”

Mr. McLaughlin began to stretch. He has been filming “Shooting Stars,” a LeBron James biopic, and had just managed his first slam dunk and he could still feel it in his hamstrings. (Mr. McLaughlin is 5-foot-8, so this is especially impressive.)

Mr. Ristic had him practice a typical forehand swing and Mr. McLaughlin obliged, before politely revealing that he is in fact left-handed. Switching grips, he practiced again — drop and swing, drop and swing. Then Mr. Ristic moved to the backhand.

“Oh that’s beautiful,” Mr. Ristic said, praising his two-hand swing. “Perfect, awesome. You sure you need lessons?”

“You think I could become pro?” Mr. McLaughlin said.

“Definitely potential.”

Mr. Ristic had him face away, turning to hit the ball only when commanded. Then he fed him two balls at once. Then they practiced a few lobs.

“It’s a little like dancing,” Mr. McLaughlin said, reaching with a graceful extension of his arm. “It’s feeling good.” He hit the ball at the center of the racket, again and again, making the sound he loves: thwop, thwop, thwop.

He hadn’t had much training as a child, but even after so many years away, his natural athleticism brought the skills back quickly. He looked at ease and confident.

“I’m going to push you harder now,” Mr. Ristic said. “The better we do, the harder we go.”

On “Stranger Things,” Mr. McLaughlin’s character, Lucas, has faced trials more fearsome than two balls at once — demogorgons, mind flayers — with little more than a slingshot. (And, sure, OK, an ax.) “He’s experienced certain things that a lot of kids will never experience,” Mr. McLaughlin said meditatively. “There’s definitely a lot of trauma that plays in that.”

Still, he feels close to Lucas. “Lucas is me,” he said. “I brought him to life. He’s a friend. He’s a brother, a loyal person. He’ll go out of his way for anyone that he loves.”

When he was hired for the show nearly seven years ago, he was 13, playing 12. “I was adorable!” he said. He grew up on camera. He even had his first kiss on camera.

“It’s a flex,” he said.

But now he is a mature man of 20, playing 15. Is it ever hard to return to a teenager’s mind-set? “No, no. I feel like I’m still a teenager.” he said. “It’s not hard to just dive right back into that childlike behavior. Especially when you’re around people that you grew up with. It’s not hard to remember those good times.”

And yet, maturity has pushed him beyond acting. He has recently released two neosoul singles, “Neighborhood” and “Soul Travel,” recorded independently, and he is learning about financial literacy, including budgeting and investing.

“The 20-year-old Caleb’s evolving, he is coming into more of who he is,” Mr. McLaughlin said, speaking in the third person. “I just want to expand my learning and become a better person.”

At the moment, his focus was on tennis. After nearly an hour of lessons, Mr. Ristic challenged him to a game, best of seven points.

“Let’s go. Let’s put everything to work,” Mr. Ristic said. “No mercy.”

“No mercy,” Mr. McLaughlin agreed. “I’m ready.”

They began to play, though Mr. Ristic may not have been entirely merciless. He returned Mr. McLaughlin’s shots, even the ones that went wide. And when Mr. McLaughlin couldn’t quite answer a serve, Mr. Ristic served again.

“Am I winning?” Mr. McLaughlin asked, incredulous.

“It’s 3 all,” Mr. Ristic said.

They played a few more points and Mr. McLaughlin took the lead.

“I’m just too good,” he said.

“Lucky shot,” Mr. Ristic said, as another ball whizzed past.

“I got you on your feet,” Mr. McLaughlin crowed.

Mr. Ristic scored, with a ball wreathed in top spin. But then Mr. McLaughlin finished the match with a winning lob. (Had Mr. Ristic perhaps let him win? Unclear.)

“I forgot how much I miss tennis,” Mr. McLaughlin said, as they congratulated each other. “I’m sweating a lot. And you went easy on me.”

As they collected the balls, Mr. Ristic urged him to return for more training. “You’re a natural,” Mr. Ristic said.

Mr. McLaughlin, smiled, ready to add another hyphenate to his dancer-singer-actor-dunker résumé. “I try, I try, I try,” he said.