The 29-year-old has struggled with a knee problem in the build-up to the tournament and limped out of Portugal's training session on Thursday.
He was seen leaving the pitch with an ice-pack on his knee, but he has insisted that he would be unwilling to play if he did not feel that he was ready.
"I would be the first to tell the coach if I were unable to play, I would never put my career at risk even for a World Cup," ESPN quotes him as saying.
"I'd love to be 110% and but I am 100% and that is enough to help my team."
Ronaldo is Portugal's all-time leading scorer with 49 goals in 111 appearances.