The singer-songwriter is very proud of his girlfriend's new material, despite their very different musical styles, and thinks it is going to be a ''megaton bomb'' hit.
The 35-year-old star told Billboard.com: '''Roar' is in my life. 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.''
John, who has previously dated a string of high profile women, including Jennifer Aniston, Jessica Simpson and Taylor Swift, also opened up about his new duet with Katy, 28, ''Who You Love', which features on his upcoming album 'Paradise Valley' and admits the lyrics are extremely honest.
He said: ''In 2013, modern-day times that's sweet. Because there's so many other things to occupy your time that you can do for yourself. That to say, I tried to be alone but I can't. This is sort of our generation.''
He added: ''I immediately thought about Katy and I was like, what a cool voice. What a cool artistic personality to have on. It was a really fun opportunity for her to write like, her answer to that. So you get these two sides to this relationship that are brutally honest but no less universal.''
The BBC drama starring Aidan Turner returns to BBC One on September 4th.
Audiences may be divided over whether this comedy crosses the line as it looks for...