Ronaldo was again outstanding for Real in yesterday's 2-2 draw at Barcelona.
Van der Sar said: "Cristiano told all the United lads that he wanted to become the best player in the world and he was going to make sure he would be.
"He was always the first one in at the training ground. He worked harder than any other player.
"He was training hard, he would stay on the training ground for more free kicks when the others were finished.
He would go back in the gym and he would work on his physique all the time. He has become so strong, has so much pace, because he has invested all his time and energy in that.
"He still does not go out much, he does not smoke and he does not drink."