The music mogul denied reports that Spears was "coming undone" during taping sessions for the show - claims which were also denied by the singer's publicist late last month.
Cowell told X17 Online: "No she's not unstable, I'm the one who's unstable!
"She's working out great, I mean, all these reports that we've fallen out, it's nonsense. She's doing a great job."
Cowell later added when asked if he was enjoying working with Spears: "Do you know what? Like I said, she's been amazing. She's really taking time to learn how to do this. She's got her confidence up.
"Once people give her a chance, I think people are really going to get her. She's gonna be a big surprise. She's a tough judge."
When pressed as to who his favourite judge of all time was, Cowell also quipped: "Yeah, I'd say Simon Cowell is a great guy. He's good to work with. Very good looking."
Cowell missed the latest taping sessions for The X Factor USA in Kansas City due to illness, and was replaced by long-serving X Factor UK judge Louis Walsh.
Photo gallery: 'The X Factor' USA judges in Kansas City: