Bono Sings Cabaret At New York's Carlyle Hotel (PHOTO)

U2 lead singer Bono is used to performing in stadiums holding tens of thousands of people. But Sunday night the rockstar jumped onstage to harmonize with singer Jim Caruso and pianist Billy Stritch at the intimate Bemelmans Bar at the Carlyle Hotel in New York City.

“Bono walked in. It was impossible not to recognize the guy -- the glasses alone gave him away!” a surprised Caruso told The Huffington Post.

“They were seated in the corner, where they rocked, bopped, swayed and snapped. He kept giving me the thumbs-up when he'd like a tune, and singing along quietly. “

Jim and Billy were performing standards from the Great American Songbook, of which Bono knew the words to every song. But before he made his exit, Bono made a beeline for the piano and started to harmonize with Billy Stritch on "I've Got You Under My Skin," which he last sang in 1993 as a duet with Frank Sinatra.

“The room was stopped cold,” Jim said. “Just another miraculous Bemelmans moment! After the last note was crooned, Bono flew out the door, waving to everyone. Billy looked at me and said dryly, "Well ... that happened!"