The FIFA World Player of the Year has not played since the beginning of this month due to knee and hamstring strains but is back in training and will now undergo a more vigorous session on Wednesday morning including sprints which will determine whether he does start.
"He has been fine in training and we will have another test but it is obvious he will play if everyone is sure that he doesn't have any problems.
"We are confident that he will start but a decision will be made tomorrow," Ancelotti told a news conference.
"If there is a risk for him then he won't play. This is a tie and so nothing will be decided here but hopefully we will take an advantage into the return leg."
Except for a defeat in the second leg of their quarter-final tie with Borussia Dortmund, which did not prevent them from progressing, Real have been in good form in Ronaldo's absence with two wins in La Liga and victory last week in the King's Cup final against bitter rivals Barcelona.
Gareth Bale came to the fore in the cup with a fine winning goal and he is also fit to play having overcome a heavy cold.
"He is in good spirits. He had a small problem yesterday but now he is looking forward to playing. With the game he had against Barca it is logical that it will have given him more confidence," said Ancelotti.
"At the start of the season he was not ready but gradually he has found his form through training. He has been playing well and scoring goals and I think that he will continue to progress next season.
"It will also be beneficial for him that he doesn't have to play in the World Cup."
Real are now aiming for their first Champions League final since they won the competition 12 years ago against Bayer Leverkusen in Glasgow and the pressure has been growing on the team to win their tenth crown.
"The team has confidence as at the start of the season and now we are also playing well. We want to win as much as anybody and claim the tenth which I see as an advantage for us and not an obsession. We will do what we can to win," said Ancelotti.
This is now the fourth consecutive year that the club has reached the semi-finals with the three previous times ending in failure to reach the final.
"The objective is to get to the final and we are not there yet as we have to play 180 minutes against a very strong side," Ancelotti said.
"The semi-finals are not the aim for us but to achieve that over four years isn't bad," he added.
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