The two musicians sang her song “Deja Vu” together, as well as duetting Joel’s 1983 classic hit “Uptown Girl.”

Following the surprise performance, Rodrigo took to Instagram to share some footage of the show and a selfie with the Piano Man.

“I sang with Billy Joel at Madison Square Garden today are you kidding?” she captioned one story.

In a later story, the 19-year-old captioned her and Joel’s selfie: “Biggest honor ever, still crying, thank you Billy!”

One of Rodrigo’s break-out hits, “Deja Vu” references Joel’s “Uptown Girl” with the line: “Play her piano, but she doesn’t know/That I was the one who taught you Billy Joel.”

Before the two stars performed a rendition of both songs, Rodrigo greeted fans with: “Hey, guys! Thank you so much for having me, Billy. I’m such a huge fan, and I kind of wrote this next song about you.”

The performance was well-received, with Rodrigo’s guest appearance receiving high praise on Twitter.

“Olivia Rodrigo just showed up at Billy Joel’s concert. This f***** rules,” read one tweet.

Another added: “Olivia Rodrigo going from mentioning Billy Joel in her song to performing with him?? I’m so proud of her this is crazy.”

Rodrigo played the Glastonbury Festival earlier this summer where she performed a duet with British icon Lily Allen. At the time, the Supreme Court had just overturned the landmark Roe v. Wade decision.

Before breaking into Allen’s iconic track “F*** You,” Rodrigo gave an emotional speech to the crowd in which she said women and girls will die as a result of the rollback of reproductive rights.

“This is actually my first Glastonbury and I’m sharing this stage with Lily, this is the biggest dream come true ever,” she said. “But I’m also equally as heartbroken about what happened in America yesterday. I’m devastated and terrified. So many women and so many girls are going to die because of this.”

Rodrigo then dedicated the song to the five conservative Supreme Court justices who voted in favor of overturning Roe v. Wade — Samuel Alito, Clarence Thomas, Neil Gorsuch, Amy Coney Barrett and Brett Kavanaugh.

“I wanted to dedicate this next song to the five members of the Supreme Court who have shown us that at the end of the day, they truly don’t give a s**t about freedom,” she said. The song is for the justices: Samuel Alito, Clarence Thomas, Neil Gorsuch, Amy Coney Barrett, Brett Kavanaugh. We hate you! We hate you."