During an interview with Xtra Factor host Caroline Flack, Willoughby revealed that Arthur's Motown performance of Marvin Gaye's 'Let's Get It On' had wooed her.
"It was so good. When he performed 'Let's Get It On' - I was watching it going, 'I now really fancy you' - it was amazing!" said Willoughby.
Flack added: "In my opinion, his performance was one of the best X Factor performances I've ever seen.
"When I found out that he was doing that song, I knew that he was going to nail it and I was really excited all week for Saturday to see him do it."
Watch James Arthur's performance of 'Let's Get It On' below:
James Arthur, Christopher Maloney, Union J and Jahmene Douglas are the four remaining acts hoping to triumph on this year's X Factor final.
The X Factor semi-final airs on Saturday, December 1 at 9pm on ITV1.
Photo gallery - X Factor results show 8:
Copyright: Ken McKay/Thames/Rex Features