Crooner John Mayer has given his musician's opinion on Katy Perry's new single, 'Roar'.

Speaking to Billboard about his girlfriend's comeback offering, the 'In Your Atmosphere' hitmaker explained that he heard it while it was still a work in progress, but instantly knew it had the potential to be "massive":

"I remember hearing it, going like, 'Wow what an interesting experience' to hear something that you identify as massive before it's massive. I thought it was a really sweet, unforgettable sort of experience to hear someone's work while they still have a very nascent relationship with it."

Mayer added: "In a way you get swallowed up by it, it's so big. Because it's such an incredibly big song that it doesn't need you to tell anybody, while you're eating lunch, [that] millions of people are going to be dancing on tables to that."

Meanwhile, Mayer and Perry recently recorded a duet called 'Who You Love' that will feature on his upcoming new album and it debuted this week.