Simon Cowell: Britney Spears Almost Didn't Make It A Day On 'The X Factor'

The Great Britney Spears Experiment nearly backfired on Simon Cowell - quite literally, from Day One.

With Cowell's second U.S. season of "The X Factor" bowing Wednesday on FOX at 8 p.m. ET, the executive producer and judge of FOX's "American Idol" heir apparent opened up to TMZ recently about shepherding the notoriously combustible blonde pop icon.

To hear the one-time face of "Idol" tell it, Spears' $15-million, single-season deal nearly took less than a day to blow up in the Brit's smug mug. Point in fact, Cowell admits that his "X Factor" spokespeople "half-lied" when they explained to media this past May that Spears left the audition set early the first day simply to find a bathroom.

"No one is going to admit that on the first day your highest-paid star after 20 minutes has walked off set," Cowell admitted. Actually, Spears "had a moment on the first day where she said to me 'I'm not sure I can do this.'"

This season, Spears is set under reconfigured contestant categories to guide the performances of both male and female soloists ages 17-24. Cowell claims that since that Day-One-jitters chat, she's become increasingly comfortable in the role.

It's a good thing, too. Between "Idol" and "X Factor," the competing prime-time races to a recording contract are spending a ton of money burning holes in FOX's pocket - not a small amount, in the name of each competing with the other. Mariah Carey's $18-million deal to judge the twelfth season of "Idol" could hardly be viewed as anything less than a response to Cowell nabbing Spears first. For Spears to have a meltdown of historic (even for her) proportions after Cowell has invested so much to give himself a leg-up on "Idol" would be to lose an irrevocable degree of face.

Not to worry that nothing's keeping Cowell on his toes. He's fighting mad that "The Voice" creator Mark Burnett and NBC executives shirked a reported "gentleman's agreement" that NBC's premiering third season of "The Voice" wouldn't go head-to-head with Cowell's "X Factor" premiere, opting instead to expand the NBC hit to three-night premiere event that concludes by indeed infringing upon the Wednesday-night "X Factor" time slot.

Cowell tells TMZ that he's bouncing back with a brand-new ad campaign with a fresh, Harvey Levin-penned slogan.