Ronaldo is widely regarded alongside Barcelona's Lionel Messi as the finest player in world football, but Casemiro believes that the Portuguese stands alone.
"Cristiano is the best player in the world. He is really something to see and is an example for the younger players," he told UEFA's official website.
Casemiro featured as a second-half substitute as Madrid booked their spot in the quarter-finals of the Champions League with a 3-1 win over Schalke 04 at the Bernabeu on Tuesday.
Ronaldo's two strikes in that particular match took him onto 13 Champions League goals for the season, just one short of the record for one campaign, which is currently held by Messi.