Los Angeles, California - Billie Eilish and Hayley Williams showed off their impressive vocals with a killer surprise duet performance of Paramore's All I Wanted.

The 21-year-old made an unannounced appearance at the band's Los Angeles concert at the Kia Forum on Wednesday night.

Eilish and lead singer Williams belted out the 2009 Paramore hit together for an encore like no other!

On Thursday, the Ocean Eyes artist praised Williams and the band in an Instagram post that featured photos and videos from the performance.

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Proving her fangirl cred, Eilish also included a clip of her childhood self performing an acrobatic routine to the same track.

"wish i could tell tiny me," the Grammy winner wrote in the caption.

In true emo fashion, Eilish rocked an oversized black dress shirt with a striped tie, black shorts, and a matching pair of black Converse for the concert.

While Eilish is fresh off her new release from the Barbie soundtrack, 34-year-old Williams was recently featured on a new vault track from Taylor Swift on her latest re-recording, Speak Now (Taylor's Version).

Paramore, who opened for Swift on the first two nights of The Eras Tour, was recently confirmed as the opening act for the upcoming UK and Europe leg as well.