The unexpected appearance marked Williams' first time on the Coachella stage.
She and Eilish performed a high-energy duet of the 2007 Paramore hit "Misery Business." As Williams, 33, proved more than a decade has nothing on the timeless revenge anthem's appeal, Eilish easily sailed through a series of beautiful, breathy harmonies.
The performance came as an especially rare treat because Williams has publicly distanced herself from the song in the later years of her career, citing certain lyrics she interprets as problematic, namely: "Once a whore, you're nothing more/I'm sorry that will never change."