The 49-year-old said that she and the former Pussycat Doll, who replaced Cheryl Cole on the show, know more about music than he ever will.
She told The Sun, 'He's going to wish that he never hired me.
'Nicole and me are to be feared because we are fierce. We are performers. We understand the spirit and psyche of artists.'
But she admitted that despite her competitive streak, she is chuffed to be working with her old American Idol sparring partner again.
'Believe it or not, we became really good friends - especially when we talk on the phone. It's just that when we get in front of the camera it brings out the worst in both of us.
'But after nine years of working together, I really got a sense of understanding the complex man that Simon is.
'And he has had the chance to really understand me. We're both passionate people.'