A fan-shot video captured the start of the performance, with Mendes delivering a stripped-down version of the track backed only by a keyboard. Halfway through, Sheeran snuck on to the stage and picked up the song – though in the clip, his vocals simmer well beneath the roar of the audience. Mendes soon rejoined Sheeran and even though they were both only playing acoustic guitars, the pair managed to bring "Mercy" to a mighty, arena-sized end.
Mendes has cited Sheeran as a major influence on his music and career. When Mendes was first starting out as a musician, posting songs on YouTube and Vine, he frequently covered Sheeran, but soon looked to the pop star for other inspiration.
"Ed Sheeran wrote his songs, so I wanted to write my own songs," Mendes told Rolling Stone last year. "He's just like an average guy and a very approachable person. I've actually gotten the chance to meet him, and he's just like a very sweet guy. He is just so normal, and that's the best part about him."
Mendes released his most recent album, Illuminate, in 2016. His current North American tour wraps August 23rd at TD Garden in Boston.