Katherine Heigl pretended John Mayer friendship was much more to test boyfriend

New mom Katherine Heigl once used pal John Mayer to test her romance with Josh Kelley.

The Knocked Up star had been dating the singer for a month after they fell for each other on a video shoot, but she wasn't sure where the romance was going and so she decided to see if her new man would fight for her.

During an appearance on The Late Late Show on Tuesday (14Feb17), Katherine told host James Corden, "John and I grew up in neighboring towns and we kind of knew some of the same people and had friends in common. I think we met at one of his concerts and (became) friends. Just friends.

"Josh and I were dating about a month and it was unclear where the relationship was going... and I didn't know if he was as serious about me as I was about him, and I had run in to John, who was in town, making a record... so were hanging out, having drinks, having dinners, as friends... He wasn't interested in me in that way... but Josh didn't know that."

Katherine admitted Kelley, her husband of almost 10 years, still doesn't believe that she and Mayer were just friends, and she revealed she used his worst fears to make her then-boyfriend commit.

"I said, 'If this isn't exclusive and you're dating other people, that's fine, but I'm gonna continue seeing John...'," she recalled with a smile.

Katherine gave birth to the couple's third child, son Joshua Bishop, on 20 December (16). Heigl and Kelley are also parents to eight-year-old Naleigh, who they adopted from South Korea in 2009, and her adopted sister Adalaide, four.