Ancelotti, who arrived in the Spanish capital from Paris St Germain in the summer to replace Jose Mourinho, admits even he has been surprised by Ronaldo's professionalism.
In an interview with Cadena Ser, the Italian admitted the Portugal superstar had made the biggest impact on him during his short time at the Bernabeu.
He said: "Cristiano for his professionalism.
"He is very serious, looks after the small details. He motivates and helps others.
"Cristiano trains well, not only on the pitch. He is diligent and professional off the field too."
Ronaldo is likely to have to be at his brilliant best at the Nou Camp on Saturday for a game Madrid can ill afford to lose.
A Barca victory would move the Catalans six points clear of their arch rivals - a not insurmountable deficit for Madrid but one which could prove daunting at this early stage of the season.
Having confirmed Diego Lopez would return to replace Iker Casillas in goal, Ancelotti also hinted Gareth Bale would make just his second start since joining from Tottenham.
Bale has been troubled by a thigh injury but is now returning to full fitness.
Ancelotti said: "His physical condition is better and he is now ready to start a game."
If he does play, Ancelotti will be left with a decision on who forms his attacking trio.
Striker Karim Benzema has struggled in recent weeks whereas winger Angel Di Maria has blossomed.
Ancelotti has been impressed by Di Maria's attitude after he allowed Mesut Ozil to leave for Arsenal.
Ancelotti added: "I decided to move Ozil on, it was a technical decision. I prefer Di Maria for the balance of the team.
"Di Maria has less quality than Ozil but I prefer his character and (the way he) helps the team.
"With the arrival of Bale it was better to have Di Maria than Ozil."
Barca go into the game in the rare position of having not won in their previous two outings - against Osasuna (0-0) and at AC Milan (1-1) in the Champions League.
And captain Carles Puyol, who made his return from a seven-month injury absence in Pamplona last weekend, is wary of that becoming three against a Madrid side he believes are improving.
He said: "Real Madrid always demand the most from you. They're playing better and better.
"We'll have to be at our best to beat them. We know they're dangerous on the break. We know them very well."
Puyol is not attaching too much significance to Saturday's game in terms of the overall league picture.
"It won't decide the title," he said. "There's a long way still to go but it is a good opportunity to play well and gain confidence."