The long-time presenter teased on ITV's This Morning that everyone had been blown away by the unusual performance, which came at the end of a long day of auditions.
Chatting about what a contestant needs to do to impress, he said: "I think the most important thing is, if I'm brutally honest, is surprise the judges.
"So many people come in and a young girl will do a Rihanna or Adele song and you feel for them but you always want someone to take them to one side and say, 'What are you doing? You'll only get compared to Rihanna.'
"When people come in and are almost a step ahead and will rework a song [it is better]. We had a girl the other day sing 'Fresh Prince of Bel-Air' with a guitar. It was really stripped back and you wouldn't have seen it coming a mile off. It was the last audition of the day.
"We stopped filming at 1am and everyone was tired, then she came in and it was one of the best auditions of the day.
We can't wait to see it!
Auditionees will have to impress a new-look panel this year with Simon Cowell and Cheryl Fernandez-Versini returning to the show for the first time in three years. Louis Walsh is back for his 11th year in a row, while Scary Spice Mel B makes up the fourth place on the panel.
Radio 1Xtra DJ Sarah-Jane Crawford replaces Caroline Flack on Xtra Factor.
X Factor returns for its 11th series on Saturday, 30 August on ITV1.
Watch the official X Factor 2014 trailer below:
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