John Mayer & Shawn Mendes Duet on Mash-Up of 'Mercy' and 'In Your Atmosphere' in Toronto

Cue the fangirling.

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."

Both strumming acoustic guitars for the surprise collaboration, the singer-songwriter pair harmonized and switched off singing verses from each other's songs. As the Mendes Army knows, the 18-year-old has deemed the 39-year-old Mayer an inspiration since the beginning of his own career, so this was a pretty monumental moment for Mendes -- but frankly, it was probably nearly as killer for those who were lucky to witness the duet in person.

Mayer -- who offered Mendes advice for some of his Illuminate tracks -- also clearly enjoyed the tag-team performance, as he took to Instagram give the "dynamite" Mendes a shout-out. He even called their mash-up "over the top good."

Of course, Mendes couldn't go without tweeting about the special performance either, declaring his love for Mayer once more and adding that it was one insane night.