Justin Bieber locked in on Billie Eilish and delivered the moment everyone's talking about at Coachella Weekend 2 ... serenading the pop star mid-set and sending the crowd into a frenzy.

Bieber launched into "One Less Lonely Girl" and made Eilish the clear focus, turning the performance into a full circle scene between the global superstar and one of his most famous fans.

SZA joined him under the desert lights for a sultry "Snooze" duet, while Sexyy Red injected high energy into "Sweet Spot." Big Sean kept the momentum going, hopping in for "As Long As You Love Me" and "No Pressure," giving the set a rapid-fire hit streak.

The stacked lineup was a sharp pivot from Bieber's stripped-down Weekend 1 performance, which drew mixed reactions. This time, he came with spectacle, surprise and star power ... and the difference was night and day.

He also wove in tracks from "SWAG" and "SWAG II," mixing new material with throwback hits to keep both longtime fans and newcomers locked in.