Olivia Rodrigo brought out Alanis Morissette for her Sour tour concert at the Greek Theatre in Los Angeles on Tuesday.
Rodrigo's "favourite breakup song" was performed. Rodrigo, 19, joined Morissette, 47, for the chorus.
They sang, "I'm here to remind you/Of the mess you left."
"That's the craziest thing that's ever happened to me," said Rodrigo.
Added: "She's an angel. Rolling Stone's October Musicians on Musicians issue featured Rodrigo and Morissette.
Rodrigo's Sour and Morissette's Jagged Little Pill were both critically praised heartbreak albums.
Rodrigo opened with "brutal" and played every song from Sour.
Rodrigo spoke out between songs "hope ur alright" and "happier" on the Texas Robb Elementary School shooting that killed 19 children and 2 adults.
She ended her debut night at the Greek Theatre with "good 4 u" before confetti exploded and Rodrigo waved goodbye.
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