Simon Cowell showed off his vocal talent while attending the Denver auditions for this year’s X Factor USA.

Simon was less than impressed with one candidate’s version of 1940s classic ‘When Fools Rush In’ and fellow judge Kelly Rowland encouraged him to try his own version.

Simon branded this particular day as the “worst” day of auditions ever and he clearly felt it was time to show them how it’s done.

The music mogul has always vowed to never sing in public but he obliged Kelly’s request. He was praised by his fellow judges with Demi Lovato saying his performance was charming.

Kelly added: “I knew that voice was always there.”

This year's X Factor USA judges: Simon Cowell, Paulina Rubio, Kelly Rowland and Demi Lovato

The episode featuring Simon’s singing will be screened in the US tonight (12.09.13) and ITV2 will show it on September 20.

Simon recently appeared on the Jimmy Kimmel show talking about impending fatherhood and it seems the music producer isn’t likely to become a hands-on dad.

When asked if he would get up early Simon replied, “Don’t you pay people to do that? I will probably do it a couple of times, I was brought up by nannie, I didn’t recognise my mum until I was about 3 or 4.”

Cowell also admitted there were rumours he has fathered other kids but “this is the first official one.”