The actor told Parade that he suggested a number of other stars to play the part of Guido Contini in the Federico Fellini-inspired movie.
Day-Lewis said: "I sang as a choirboy in local church when I was a schoolboy, but other than that, I hadn't done any singing to speak of.
"[Director] Rob Marshall convinced me, really against my better judgement, that I would be able to pull it off.
"I really tried to think of every excuse I could not to do it because I felt he needed somebody else. I think I gave him a few names actually, but he kept saying, 'No, I think you can sing'."
He added: "I was nervous as hell. What gives you courage is you're playing a character who's singing. Without that, you'd have nowhere to hide.
"We all worry about the quality of our voices if we're not singers. We're worried the whole time about the sound that we're making which finally, although it's not irrelevant, is not the most important thing."
> Day-Lewis 'lacks dancing confidence'