The Liga outfit pulled ahead through Sergio Ramos and Karim Benzema in the Club World Cup semi-final, but the former was lucky to avoid a red card when he brought down Mariano Pavone in the penalty area before half-time.
Casillas kept out Gerardo Torrado's resulting spot-kick - his second penalty save in as many matches - before Gareth Bale and Isco went on to seal a convincing victory.
"It is true that the penalty save was key," he said. "With the game at 2-0 it looked finished and they had a chance to get back into it.
"It is never easy, but the team played well and won. Now we'll see what the final throws up.
"We had strength, organisation, stayed focused and we did our best."
Although Cristiano Ronaldo failed to find the net himself, Ancelotti was satisfied with the attacker's performance after he provided the assists for both Bale and Isco.
"He played very well and gave assistance," he continued. "He is always very effective, even though he did not score.
"He is surely saving a goal for the final."
Real will meet the winner of Wednesday's clash between San Lorenzo and Auckland City in the final.
"San Lorenzo have more experience than Auckland, but in football anything can happen and we will prepare well for the final whoever the opponent," added Ancelotti.
And Real will be boosted by the news Sergio Ramos should be available for the final, despite being withdrawn shortly after the hour mark on Tuesday.
"I do not think it [the injury] is worrisome," the Italian coach explained. "In two days he can be good and available for the final."
