Real had to come from behind to see off David Moyes' side at the Santiago Bernabeu after Aritz Elustondo had opened the scoring inside the first minute.
France striker Benzema struck twice in the second half after James Rodriguez and Sergio Ramos had put the leaders in front at the break.
Real were without the suspended Cristiano Ronaldo, who sat out the first game of his two match-ban for a red card against Cordoba, and Ancelotti was pleased to see Benzema take centre stage in the Ballon d'Or winner's absence.
The Italian said: "Benzema put in a fantastic performance. The goal he scored was top quality. He has a fantastic strike, but we also have to appreciate the way he helps the team's play.
"He isn't just a striker who plays in the box, he is very good in every area and has a lot of quality. We had a lot of chances and shots on goal."
It was the third time in recent weeks that Real have fallen behind to an early goal and that is clearly a cause for concern for coach Ancelotti.
He added: "In the first half, as has happened in other games, we didn't start well and we conceded a goal. The same happened at Cordoba and against Atletico (Madrid) in the Copa (del Rey).
"We have to improve that. Then we defended well and we pressed up front. We played very well in the second half. We sorted it out and the match improved."
Ancelotti also revealed that new signing Lucas Silva did not feature as he is not yet match fit.
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