Musician John Mayer has heaped praise on his girlfriend Katy Perry's new single, insisting he knew Roar would be a hit the moment he heard it.

The I Kissed A Girl hitmaker premiered her new song on Monday (12Aug13) after it was leaked online over the weekend (10-11Aug13).

Critics have already predicted the track is destined for chart glory, and Mayer, who has been dating the singer on and off since last year (12), insists he was blown away when Perry first played it for him.

He tells Billboard, "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.

"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."

Mayer has also teamed up with Perry in the recording studio - they have collaborated on Who You Love, a country-style track which will feature on the rocker's upcoming album Paradise Valley.