Olivia Rodrigo wasn't alive when Billy Joel was making his hits -- but she clearly appreciates the piano man's work, as she joined him onstage for one of his classics, and one of hers! #Respect
Sure, he could easily be her grandfather, but Wednesday night Billy and the 19-year-old singer rocked together at Madison Square Garden ... where he's had a residency for almost 9 years! They performed "Uptown Girl" and then followed up with her hit "Deja Vu" -- a song where she name-drops Billy.
While the crowd was definitely older than Olivia's usual audience, fans went wild when she took the stage -- even singing along to her track, which dropped last year.
Olivia's the latest surprise guest Billy has welcomed over his years at the Garden -- he's also brought on Miley Cyrus, Jon Bon Jovi, and Tony Bennett ... to name a few.
His MSG show came in the middle of Harry Styles' 15-show run at the legendary arena, and Billy playfully thanked the singer for movin' out, saying ... “I’d like to thank Harry Styles for taking the night off, so we can kind of squeeze in here.”