It’s been “party over, oops, out of time” for about a quarter of a century now, but Troye Sivan and Charli XCX are still partying like it’s “1999” — their version, not Prince’s. During Sivan’s concert at London’s OVO Arena Wembley, Charli XCX made a surprise appearance, not that the audience was ready for it.

Sivan introduced the single, which the artists originally released in 2018, with its chorus, and the crowd sang along. Then the lights went out, and Charli XCX’s silhouette appeared. She took things over from there, and the audience ate up the nostalgia, singing along with the chorus about going back to 1999 just to sing “Hit Me Baby One More Time” one more time.

Sivan first met Charli XCX at a Los Angeles house party. “We got talking and realized we had a lot of mutual friends,” she said in 2018. “Immediately, I thought he was super sweet.” When she played him her demo for “1999,” saying she wanted to give it to another artist, he pleaded with her to change her mind. “He said it was pretty cool, and if Troye thinks it’s cool then it’s cool,” she said. “He has pretty good taste.” Her choice of the title “1999,” for what it’s worth wasn’t for nostalgia, she said, but because it was “really poptastic.”

At the London concert, Sivan welcomed another collaborator onstage earlier in the show: Ross Lynch, who performed “One of Your Girls” with him. The performance began with a phone call on the big screen behind the stage, and then the real show began. Lynch previously appeared in the video for the single, which came out on Halloween.