Watch Lady Gaga Make Surprise Appearance With U2 at MSG

Singer joins group for 2013 Nelson Mandela tribute "Ordinary Love"

U2's ongoing Innocence + Experience Tour has featured unannounced guest appearances by everyone from Jimmy Fallon and the Roots to an Elvis impersonator and U2 cover band. But Sunday night at Madison Square Garden, the biggest star yet came onstage when Lady Gaga joined them for "Ordinary Love." Rumors of her appearance spread across Twitter hours before showtime, but ripples of shock still shot through Madison Square Garden when Bono asked if there was a piano player in the audience and Gaga, wearing what could be best described as underwear, walked onstage.

"Oh my goodness gracious," Bono said. "It's yourself. Her Ladyness, her Gaganess is here this evening. That's a treat. This is a song called 'Ordinary Love' written for the great Nelson Mandela." Gaga didn't say a word to the audience before getting behind the piano and joining the group on the tune written for the 2013 Nelson Mandela biopic Mandela: Long Walk to Freedom. She was wearing enormous Alexander McQueen boots, so The Edge gallantly helped her walk off the stage when it was done. "Wonder Woman of New York City here with us tonight in Madison Square Garden," Bono said. "What a delight."

Later in the night, Bono sang a brief snippet of "Born This Way" at the end of "Beautiful Day." It was otherwise a normal stop on the tour, though they did return "With or Without You" to the end of the main set after briefly experimenting with moving it a few songs earlier in the night and wrapping up with "Pride (In The Name of Love)." They closed the show with "One," and Bono once again allowed the audience to sing the vast majority of the tune.

The tour resumes Monday with the band's sixth show at Madison Square Garden, with the U.S. leg wrapping up with two more MSG shows on Thursday and Friday.