Not many people get applause just for showing up to work, but for Andy Karl the audience response was thunderous when the star of the new musical “Groundhog Day” — who’s been out of the show since injuring his knee mid-performance Friday — stepped back onstage Monday for opening night.

The culprit — a torn ACL, “just one of those crazy flukes that can happen . . . [in] a physically demanding show,” he explained on Instagram Monday, adding “ ‘The show must go on.’ ”

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And it did, with Karl, 43, who earlier this month won an Olivier Award (England’s Tony) for this role, wearing a black knee brace and looking pain-free — till about 20 minutes in, when he started limping and favoring his left knee slightly.

“Oh my God,” he mouthed to the crowd at his curtain call, all smiles.

Karl was required to perform opening night to be eligible for a Tony nomination, and must continue performing if voters are to see him.

The musical—with score by “Matilda” composer Tim Minchin, book by Danny Rubin—is based on the 1993 Bill Murray film about a sarcastic weather forecaster reliving one day over and over . . . until he gets it right. It’s a premise Karl, who says he’ll recover with rest and physical therapy, probably understands now more than ever.