For nearly 40 years, Sylvester Stallone has been indelibly connected to Rocky Balboa. He created the underdog South Philadelphia boxer and starred as him in six Rocky films, the first of which won the Academy Award for Best Picture.

Now, another actor has stepped into the fictional boxer’s trunks for the Rocky musical at the Winter Garden Theatre: Andy Karl, a Broadway veteran (9 to 5, The Mystery of Edwin Drood) who makes a boxing and singing Rocky come to vivid life.

Photo: Matthew Murphy

And yes, even die-hard fans of the original can be confident that this translation from groundbreaking film to epic stage musical arrives on Broadway with Stallone’s imprimatur. Besides his involvement as a producer, he is part is of the creative team, co-writing the book with Thomas Meehan (The Producers).

Their script, mirroring the plot of the movie — including its South Philly working-class backdrop — deftly tracks the fading club boxer who collects debts for a loan-shark to his big break as the unlikely opponent of the world heavyweight champion, Apollo Creed (a wonderful, braggadocious Terence Archie), at a New Year’s Day 1976 match.

In fact, the musical is so faithful to its cinematic antecedent that in training for the mega-match, Rocky spars against slabs of meat and bounds the many steps of the Philadelphia Museum of Art to the iconic strains of “Gonna Fly Now.”

The show also offers a poignant and lovely portrayal of Rocky’s bumpy courtship of shrinking violet pet-store clerk Adrian, played by Margo Seibert in her Broadway debut.

Andy Karl and Margo Seibert. Photo: Matthew Murphy

As for Stallone’s take on the show’s casting, in an early interview with the Associated Press he had nothing but high praise for Karl’s approach to Balboa.

“He has what it takes. There’s no arrogance; there’s a natural humility about him and that’s what is important,” he said, adding, “No matter how threatening he may look, you’re going to like him. It just comes through. And that’s not easy to find. Tough guys are a dime a dozen; a sensitive tough guy, pretty rare.”

And while Karl has the mandatory buff Rocky physique, he also has something Stallone did not: songs. The production features an inspirational score by Lynn Ahrens and Stephen Flaherty, whose Broadway collaborations include Ragtime and Seussical. The team’s skill is evident in both company numbers and show-stopping turns by Rocky, Adrian, Apollo, and Mickey (Dakin Matthews), Rocky’s trainer.

During the archly optimistic “My Nose Ain’t Broken,” for example, as he ices his swollen eye, Rocky sings: “I got 10 sore knuckles and a ringin’ ear / I got a bruise over here and here, and over here / I’ve got a swelled up eye and a real flat beer / but hey, my nose ain’t broken.”

As in the film, the Rocky-Adrian love story propels the show. Clearly they are broken souls who find themselves in each other — and it’s her love that gives him the confidence to face Apollo at the Spectrum in Philadelphia.

Photo: Matthew Murphy

The final scene — the fight — is a lollapalooza. Theatregoers who occupy the first several rows of the center orchestra are ushered onstage to tiered arena seating; a boxing ring moves out into the audience…  announcers sit on a perch overlooking the action… and the bout (brilliantly choreographed by Steven Hoggett) begins in earnest.

For the next 20 minutes, the line between acting and athleticism blurs as Karl and Archie clash and the Winter Garden and its audience morph into the Spectrum arena nearly 40 years ago.

“I think people start thinking, ‘Am I at a sporting event?’” Seibert said when she and Karl appeared on Today in early April. “People are screaming and standing up.”

Rocky is playing at the Winter Garden Theatre, 1634 Broadway (50th-51st Sts.) For tickets, call 212-239-6200 or click here.