Thomas Rhett had a cross-continental, cross-genre collaboration in store for fans when he took the stage at London's O2 Arena for a performance at C2C Festival on Friday night (March 10.) He brought pop singer-songwriter Niall Horan to the stage during his set, and the pair performed a duet version of Horan's 2017 hit, "Slow Hands."

It was a song that was very fresh for Rhett and his band, he admitted from the stage as he introduced Horan to the crowd. "Me and the band learned this song yesterday," he said. "And we rehearsed it this morning. So we're gonna give you our best shot."

From there, he welcomed his surprise guest to the stage, and the crowd roared when Horan walked out. Though the song was fresh for Rhett, he appeared to handle it flawlessly, trading verses with Horan over a pulsing, R&B pop beat. After they concluded their duet, Rhett bowed down to Horan and the two shared a hug.

Rhett and Horan come honestly by their friendly onstage banter: They've long been buddies, and have even been a couple writing rooms together, according to a spring 2022 report from Music News.

"I feel like every time me and Niall are together we threaten to do some sort of collaboration. We have written a couple of times but I don't think we have written the right song yet for us to do," Rhett told the outlet, adding that he thinks there's a solid chance that fans could see a duet between himself and Horan come out in the next "couple of years."

But for now, Rhett described his C2C team-up with Horan as a "bucket list" moment, adding on Twitter, "Love ya bro! Thanks so much for joining us in London last night!"

Horan has recently added a little extra country flair to his life as a coach on Season 23 of The Voice, where he's on set alongside Blake Shelton. The veteran country coach is currently in his last season on the show before retiring; Shelton has been on The Voice for every season since the show's inception.

Thomas Rhett's Best Songs Tell His Story - See the Top 20

Thomas Rhett has released some unforgettable songs over his career. Here are 20 of our favorites.

More From K99