The Spanish champions host the Serie A holders on Wednesday with four points separating the sides after just two rounds of matches, but Ancelotti insists they will face a stern test to record a win at Santiago Bernabeu.
"Juventus are very motivated and very well organised, with quality players," the 54-year-old was quoted as saying on the official Uefa website.
"They've signed some important players in the summer, like [Fernando] Llorente and [Carlos] Tevez who have given the team some extra quality.
"They have played together for three years under the same coach and it's a well-structured club. Those are their biggest qualities."
Juventus have collected two points from their opening two Group B games with Copenhagen and Galatasaray, while Madrid have recorded wins in each of their first two games.