The Italian says the 29-year-old will have a fitness test on Wednesday to determine whether he can start but has backed Isco to step up should he be fit only for the bench
The Portugal star returned to training on Monday after recovering from a hamstring injury sustained in the quarter-final meeting with Borussia Dortmund.
Ancelotti says he is happy with how the 29-year-old has progressed in training but is adamant he will not risk the attacker if he is not 100 per cent ready to return to action.
"Cristiano has trained well. We will make a decision after another test tomorrow. He will play if he is totally fit," he told a press conference.
"Ronaldo will not play if there is any risk. We have another decisive game next Tuesday [in the second leg]."
The Italian believes having Ronaldo as an option from the substitutes bench could prove to be extremely useful for Madrid on Wednesday and has backed Isco to step up and replace the Portugal captain in the starting XI.
"Replacing a player with Cristiano is not easy because everyone has done a fantastic job. Isco proved he could be important," he said.
"Cristiano's presence will give us more options on the bench."
Ancelotti added that the Copa del Rey triumph over Barcelona will give his players confidence heading into the tie but he has stressed winning a 10th European title is not "an obsession" for the club.
"We played with 11 players against Barcelona and were compact and solid. It will be the same tomorrow," he continued. "We have the same confidence as before, but our play has improved with time.
"We are in the semi-finals and we want to win the title more than anyone else. It's not an obsession, it's motivation. We have a lot of desire and are excited to do our best tomorrow.
"Failure is a bad word, but I can't say reaching semi-finals is success. The first objective is to play the final."
Pep Guardiola has never lost a match as coach at Santiago Bernabeu, but while he is full of respect for his opposite number, Ancelotti has warned that the Spaniard will not be the decisive factor in the last-four tie.
"I have a lot of respect for Pep. He has done a good job. We have to be focused tomorrow because they are a tough team. [But] semi-finals are won by players, not coaches," he added.
"We will face a very strong team. They won the league a month ago. It's normal that we are not considered as favourites.
"We don't think about [Frank] Ribery; we are focused on the whole team. He is a great player but we think about his team."