Lorde’s surprise performance at Charli XCX’s Madison Square Garden show in New York

Lorde and Charli XCX in New York at Madison Square Garden, where the pair performed their duet Girl, So Confusing. Photo / @charli_xcx

Lorde has joined Charli XCX on stage at the Brat star’s sell-out Madison Square Garden show for a performance of their hugely popular duet Girl, So Confusing.

Striding down the stage’s runway, the pair wore complementary looks from Saint Laurent and both wore their voluminous hair out – no doubt a nod to the lyric “people say we’re alike, they say we’ve got the same hair”.

The surprise appearance earned a thunderous response from concertgoers. “The sound barrier in Madison Square Garden nearly broke,” reported Rolling Stone of the “fiery” rendition.

@itsmehenri_

IT’S SO CONFUSING SOMETIMES!! @Charli XCX #lorde #msg #sweattour

♬ original sound - It’s Me Henri

Having the two stars together on stage underscores the journey the song and its subject matter has had.

The original appeared on the hugely successful album Brat, released by Charli XCX (Charlotte Emma Aitchison) in June, and fans immediately speculated that the song – which details an awkward relationship with another celebrity – was about Lorde (Ella Yelich-O’Connor).

“Yeah, I don’t know if you like me; sometimes I think you might hate me, sometimes I think I might hate you; maybe you just wanna be me,” Charli sings. “You’re all about writing poems, but I’m about throwing parties.”

This theory seemed confirmed, and the issue resolved, when the remix was dropped on June 21 – a collaboration that was revealed to be a by-product of the two hashing out their differences over text.

“Well, honestly, I was speechless when I woke up to your voice note; you told me how you’d been feeling, let’s work it out on the remix,” Lorde sings. “I was trapped in the hatred and your life seemed so awesome; I never thought for a second my voice was in your head ... Forgot that inside the icon there’s still a young girl from Essex.”

The two pop stars have been publicly friendly since releasing the single, with Lorde attending Charli’s highly publicised 32nd birthday party in Los Angeles, and now appearing at the Madison Square Garden show.

The Kiwi was pictured in other images from the event, including partying to the viral Brat hit Apple alongside Troye Sivan.

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In an interview with Vulture in August, Charli confirmed that there was nothing but “good vibes” between her and Lorde, and that the scrutiny of their tumultuous friendship had had a positive effect on their relationship. “Honestly this whole thing has brought us closer. I know that’s so f***ing cheesy. But it has.”

During a May interview with Rolling Stone, she explained that a decade ago she felt envious of the Kiwi.

“When Royals came out, I was super jealous of the success that that song got, and that Ella got. You piece all this stuff together in your brain, like: ‘She was into my music. She had big hair; I had big hair. She wore black lipstick; I once wore black lipstick.’ You create these parallels and think, ‘Well, that could have been me’,” she explained. “You get over it and then you try to figure out all the things that are unique about you and you pursue that.”

Brat has been one of the year’s most successful albums, spawning a “Brat Summer”, and its artwork was adopted by presidential candidate Kamala Harris after the British pop star tweeted “Kamala IS Brat”.

Charli is currently on tour for Sweat, a lineup of shows co-headlined with her and Troye Sivan for their albums Brat and Something to Give Each Other.

Lorde – who hasn’t released an album in over three years - is speculated to be working on new music, with a cryptic Instagram post in June leaving fans theorising that her long-awaited fourth record may not be too far away.

In May she appeared on the Talking Heads cover album Everyone’s Getting Involved: A Tribute to Talking Heads’ Stop Making Sense with a version of Take Me to the River.