Ronaldo is widely regarded as one of the best players in the current game but Kaka refused to compare the Portuguese to anyone else in modern football.
"Cristiano is the most complete player we have in football today," the Brazilian told reporters.
"The strength he has, he has the ability to decide a game is unlike any other.
"I will not compare with other players, but I see it every day and I know he has the ability and the strength."
Ronaldo has scored 38 goals in 38 appearances for Madrid this season.