Barcelona progressed from their quarter-final after a 3-3 aggregate draw over the two legs, with their two away goals from the first leg proving crucial.
The French side took the lead in the tie in the second half through Javier Pastore and had their sights set on the semi-finals at that stage.
But they were pegged back once the home side introduced Lionel Messi, who had been left out of the starting XI because of a hamstring injury, from the bench.
And Ancelotti believes the introduction of the World Player of the Year was the turning point of the tie.
He said: "Even if he is not 100 percent fit, he is always a fantastic player.
"We were good in possession and played quickly up the field.
"We didn't just defend like a lot of teams who come here, we caused them problems. But Messi gave his team confidence."