John Mayer & Shawn Mendes Duet

John Mayer is only three days into his Search For Everything world tour, but he's already had a superstar guest join him on stage.

During his acoustic set at Toronto's Air Canada Centre Monday night (April 3), Mayer surprised the Canadian audience with an appearance from one of their own: Shawn Mendes. The Toronto native teamed up with Mayer for a mash-up of his hit "Mercy" and Mayer's fan favorite "In Your Atmosphere."