Chelsea came under pressure late on when the game was won, but Ancelotti was unconcerned after his side took their goal tally to 21 from five games.
"Every game in the Champions League is hard to play," said the Italian. "We started very well in the first half, we played fantastic football but the second half was different.
"Zilina put more pressure on our defence. The back four did a good match. When you are 3-0 up it's normal to have more difficulty.
"I hope to watch my team play a fantastic game for 90 minutes. Obviously, I was a player so I know sometimes there are difficult times, the opponent can come back in the game.
"If I have to judge the performance I am very happy because it's not easy to win 4-1 away in the Champions League."
Zilina coach Pavel Hapel believes the match would have been closer had his side shown a more ruthless edge in front of goal.
"I am not disappointed with the game," said Hapel. "We played well and attacked Chelsea. We managed to score so it was OK.
"We kept up with Chelsea as much as we could but I am disappointed with the productivity. The score was a surprise.
"We didn't show the same productivity in front of goal as Chelsea did."
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