Troye Sivan and Charli XCX Party Like It's '1999' (Their Version) in London

It became a “party out, 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 crowd was ready for it.

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

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

At the London concert, Sivan welcomed another partner to the stage earlier in the show: Ross Lynch, who performed “One of Your Girls” with him. The show started with a phone call on the big screen backstage and then the actual show began. Lynch previously appeared in the video for the single, which was released on Halloween.