Billie Eilish celebrated her 23rd birthday a day early on Tuesday (Dec. 17) at the Kia Forum in Los Angeles when she invited Charli XCX up on stage for a surprise collaboration on their “Guess” remix.

The singers paired up to perform the Grammy-nominated, high-energy song from Charli’s remix album, Brat and It’s the Same but There’s Three More Songs So It’s Not for the first time live at last night’s show. And, in a Brat-appropriate touch, the Forum was bathed in the album’s signature lime green palette as sour apple-hued lights lit up the stage and green lasers blasted out all around.

The audience shouted along to every word as Billie sang, “Don’t have to guess the color of your underwear/ Already know what you’ve got goin’ on down there/ It’s that lacy black pair with the little bows/ The ones I picked out for you in Tokyo,” while Charlie danced and pumped her arms to the song’s sapphic storyline.

The two women then went bananas as the track’s booming beat revved up, hopping around the stage and amping the crowd up even more. It was wild, and clearly Charli had a blast, as evidenced by a video she posted of the moment with the caption “love u billieeeeee and hbd!!! <3”; Eilish turns 23 on Wednesday (Dec. 18).

Eilish’s Hit Me Hard and Soft album is nominated for seven Grammys next year, including album of the year and best pop vocal album. Charli is also up for seven awards, including record of the year (“360”), album of the year and best pop duo/group performance for “Guess.”

Eilish has two more sold-out nights at the Kia Forum on Friday (Dec. 20) and Saturday (Dec. 21) before she heads down to Australia in February and early March, followed by spring/summer 2025 dates across Europe.

Check out footage from the surprise collab below.