Watch Taylor Swift join HAIM on stage for a 'mashup' of 'Love Story' and 'Gasoline'

Swifties are rejoicing in seeing Taylor Swift perform during a concert for the first time since 2019.

Este, Danielle and Alana Haim were joined by the music superstar at the O2 Arena in London Thursday night as part of their "One More Haim Tour."

"I heard that my girls were playing in London at the O2 and I thought, 'I'm gonna have to see that.' And it looks like there's about 20,000 other people that also thought that," Swift said to the crowd, according to clips shared on social media. "We had a thought, that if we were to do some sort of mashup, we could possibly maybe get you to sing the loudest that you have sung all night."

The "All Too Well" singer performed a rock rendition of her 2008 track "Love Story," fusing the trio's song, "Gasoline."

In addition to praising the surprise performance, fans adored Swift and HAIM's stage outfits.

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The 32-year-old artist posted a TikTok video of her matching the group's black monochrome look, wearing the same straight-legged leather pants. Swift wore a square-neck cropped tank, while the sisters wore a triangular look.

Twitter user @zoenchanted pointed out Swift's look was similar to what she wore during her 2018 "Reputation Stadium Tour."

"Insider" producer Irene Anna took to twitter to say fans will be on edge for future surprise performances.

"No one in the taylor swift universe is ever going to know peace now that she went and spontaneously performed with haim like she really could show up any place any time," Anna tweeted.

This isn't Swift's first collaboration with HAIM. In 2015, she brought them to her 1989 World Tour. The band also made a cameo in Swift's "no body, no crime (feat. HAIM)" song, part of her 2020 album, "Evermore."

"Gasoline," the track performed at the Thursday show, was part of HAIM's third studio album, "Women In Music Pt. III," released in June 2020. The band released a remix of the song featuring Swift in February 2021.

Swift performed "Willow," "Cardigan," and "August" at the 63rd Grammy Awards in March 2021. She took home album of the year prize for "Folklore" that same night, her third time winning that category.

Swift made another live appearance at the 2022 New York University commencement ceremony, where she received a Doctor of Fine Arts, honoris causa and gave a graduation speech.

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"Hard things will happen to us, we will recover, we will learn from it. We will grow more resilient because of it," Swift said. "As long as we are fortunate enough to be breathing, we will breathe in, breathe through, breathe deep, (and) breathe out — I'm a doctor now so I know how breathing works."

Contributing: Elise Brisco

This article originally appeared on USA TODAY: Taylor Swift joins HAIM at London concert, performs song 'mashup'