Irony aside, it's left Lucy O'Byrne rehearsing her duet with the Black Eyed Peas star alone for this evening's live episode.
Each of the four finalists is set to sing with their mentors and Will is determined to go ahead, despite suffering from bronchitis.
"I’m ill.i.am right now. I got a cold and it turned into something worse," he told The Sun newspaper today. “I’ve got a bronchial infection. It’s really bad. It could affect my performance, that’s the sad part of it all.
“When you’re touring you get sick on the road. I’ve performed sick before. All you can do is be open and honest.”
Speaking earlier this week, Lucy hinted that Will.i.am may join her with some OPERA in their performance together.
Talking about blending her operatic voice with Will's rapping, Lucy teased: "I'm not allowed to talk about it too much... but he may be getting involved!"
She added: "Will has always been so supportive about my style, he seems so genuinely excited about what I do."
Meanwhile, Will suggested that he would love to be able to sign his acts from The Voice.
“The reason why Simon Cowell’s show [The X Factor] has had success and The Voice doesn’t is because Simon Cowell has a record label and that is his show,” he said this weekend. “When you have a label and that’s your show, then it’s gonna work.
“I have a label, but this isn’t my show. I’m just on it. So I can’t flex as much as I would like to flex."
The Voice final begins tonight at 7PM on BBC One.
Browse pictures of The Voice finalists 2015 left in the live shows here...