While Mendes is currently working on his third studio album, rumors have circulated regarding whether he could be cooking up some new tunes with the former One Direction member. Responding to the speculation, Mendes hopes his fans can be patient.
"It will be true one day," he says. "We're really good friends and we hang out all the time, and we are always like, 'We should write a song,' and then we're planning on writing a song -- and then we just end up hanging out!"
"It's better off that way because if there's a friendship, then the song comes easy," he continues. "So, one day, I promise! I promise, guys."
"I feel like I should be getting in front of the microphone and singing, like, asking her for advice," he gushed when ET joined them on set earlier this month. "I'm like, 'Am I really sitting here right now with her being an adviser? I'm 19 years old!' This is really crazy."
Mendes is, in fact, the youngest star to ever serve as an adviser on The Voice.
"That doesn't mean that he doesn't have great experience," Keys said. "He himself is a songwriter, a musician, a producer, and I think that's really valuable to [the contestants]."
The Voice returns with Keys, Blake Shelton, Adam Levine and Kelly Clarkson on Feb. 26 on NBC. For more on what to expect, watch below!