The Take That man told Metro that he might also perform with the singer if he gets to the X Factor final.
"If Rylan does get to the final then maybe it will be me," Williams. "I wouldn't mind. I'd love that, I love Rylan.
"I think Kylie Minogue might do a better job with Rylan because I think she's dancey and poppy but if needs be I'll turn up if they're short."
He added: "[We'd sing] 'Islands In The Stream' which is not very Rylan but I think 'Islands In The Stream' with me and Rylan would be very entertaining.
"A disco version of 'Rylan's In The Stream', you heard it here folks."
Williams had previously expressed his support for 'Team Rylan' and urged his fans to vote for him.
His bandmate and X Factor judge Gary Barlow quipped in response: "We did an album together and it's all over in one tweet. In five letters it's all over!"
Williams was a guest mentor on The X Factor last month, advising Clark and the other competitors before playing his new single 'Candy' on the results show.
X Factor results show - episode four photo gallery: