Reigning champions Madrid face Napoli in the round of 16 at the Bernabeu on Wednesday night as they look for title No.12.
However, it was Napoli who got off to the ideal start just eight minutes in when Insigne took aim from 40 yards and caught Madrid 'keeper Keylor Navas out of position with a superb effort that curled around the Costa Rica global.
Napoli, who are third in Serie A, are playing in the last 16 for only the second time.
The return match against Napoli is on March 7 in Italy.
Madrid, though, showed the response of champions to level just 10 minutes later when Dani Carvajal's cross with the outside of his right boot picked out Benzema to power home a header from close range.
Holders Real Madrid have mastered the art of winning without hitting top gear this season.
Defender Nacho Fernandez said that no players can lose at Real Madrid now that Coach Zinedine Zidane is a part of the team.More news: Sasikala: Guilty verdict ends Jayalalitha aide's chief minister bid
It was a wonderful effort from the Italian worldwide, but Casemiro, like his Madrid team, would not be outdone.
Sarri said: "With the awareness that we are going to play an away match against the champions of the world and a really strong side, we will be a bit anxious but sometimes it's a positive feeling because it gives you a boost".
Real Madrid have been on fire this season with only 2 losses this season.
It was also just what Zidane would have wanted from his team.
The momentum was fully with Los Blancos, and the exclamation point was put on their display five minutes later, when Casemiro scored an absurd, first time volley from 30 yards out. With Gareth Bale back in training but not quite ready to be used in this match, Ronaldo on his longest-ever goal drought in the Champions League and the enigmatic Benzema blowing hot and cold, the French coach will have been happy to see others chipping in.
Zidane was named as Rafael Benitez's successor a year ago and the former France global guided Madrid to Champions League glory, beating rivals Atletico 5-3 on penalties in the final. They're small, tricky players with good technique.