Cristiano Ronaldo scored twice for the hosts in the first half, either side of Fernando Llorente's equaliser, before Juventus defender Giorgio Chiellini was shown a straight red card for an alleged elbow on Madrid's goalscorer two minutes into the second half.
Ancelotti's side sit top of Group B with a perfect record of nine points from three matches and a five-point lead over second-placed Galatasaray.
The Italian admitted his side's performance was not their best, but felt the win was the most important thing.
"We could have played better, but the win is what's important to us," Ancelotti said.
"The team is confident and developing well. It's a good result. We won the game after playing well for an hour, then we slowed down.
"I liked that we played well at times and we had balance. Eventually I did not like the team because it was tired.
"The intensity has to be different; we had two clear opportunities (at 2-1) and did not take them."
Ancelotti admitted Chiellini's dismissal had a bearing on proceedings.
"The game changed," he said.
"There are five referees, then they decide to penalise Juventus and we must accept it."
Goalkeeper Iker Casillas, who returned to Real's starting line-up for the game, urged his side to build on their start and seal a place in the next round.
"We have taken a very important step towards qualification," he said.
"It was not easy, though. We have nine points after three games and now we have to try and keep going and not lose in the group stage."
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