The record 14-time winners and reigning champions eased to a comfortable win against Frank Lampard's side, who had Ben Chilwell sent off in the second half for pulling down Rodrygo Goes as he ran through on goal.
Benzema netted from close range after 21 minutes after Kepa Arrizabalaga tipped a Vinicius Junior effort into his path.
Despite controlling the game, Madrid had to wait until the 74th minute for their second, with Asensio drilling home from the edge of the box after a short corner routine.
In a battle between the last two Champions League winners, Madrid demonstrated their strength with an impressive display, while Chelsea showed why they are struggling in 11th in the Premier League.
"We are satisfied with the result, for the performance but it's not over yet, we have to fight, to sacrifice for another 90 minutes at Stamford Bridge," said Real Madrid coach Carlo Ancelotti.
"We are satisfied only for tonight, it's not over yet. We know Chelsea will try to (give) everything next week."
Ancelotti, who coached Lampard at Chelsea between 2009-2011, relied on veteran midfielders Toni Kroos and Luka Modric to pull the strings.
Lampard, in his second game at the helm since replacing Graham Potter last week, threw Thiago Silva and N'Golo Kante straight into the starting line-up after injury worries.
Kante shone against Madrid in the 2021 semis as Chelsea beat them en route to winning the trophy for the second time, and his performance was one positive for Lampard.
The French midfielder slipped on-loan Atletico striker Joao Felix through at the start, but he fired at former Chelsea stopper Thibaut Courtois.
Madrid fans had shown their support for Fede Valverde before the game, after he allegedly punched Villarreal's Alex Baena on Saturday, and he nearly rewarded them with a piledriver which flew narrowly off-target.
"For me he was the best player on the pitch, he put in a lot of energy," said Ancelotti.
"He was impeccable at recovering the ball."
Benzema, who netted a hat-trick at Stamford Bridge last season and scored the goal which sent Madrid through against the Blues in the second leg, tapped home his sixth goal against Chelsea to open the scoring.
Dani Carvajal lofted a tempting ball over the top and although Kepa got his fingertips to Vinicius's effort, Benzema was left with a simple tap-in.
Chelsea almost produced an instant equaliser but Courtois made a stunning save to deny Raheem Sterling from close range.
Madrid toyed with their opponents, penning them back in their box for large stretches, with Vinicius, Rodrygo and David Alaba going close.
Blues blows
Chelsea have struggled badly for goals this season and after their early chances, found it difficult to build in attack.
The Blues have not scored in their last four matches across all competitions for the first time since 1993 -- and their only pure striker, Pierre-Emerick Aubameyang is not registered to play in the Champions League.
"The last bit is sometimes confidence," said Lampard. "If you work, keep going, something can change.
"No player doesn't want to score a goal, sometimes they need support, confidence and a push in a positive way."
Chelsea's uphill struggle was made harder before the hour mark when Chilwell tugged back Rodrygo as he ran through on goal and was shown a straight red card.
A difficult evening was compounded when Asensio squeezed an effort through Fofana's legs and into the bottom corner to double the hosts' advantage.
Benzema might have netted another in stoppage time but headed off target after Kepa pushed a cross into his path.
Antonio Rudiger made a good block to thwart Mason Mount late on to ensure his former side could not take anything positive into the second leg.
"We're 2-0 down against Real Madrid, that's a fact now, that's done," said Lampard.
"(But) we're in a different place now, we're in a place to prove things wrong, change the tone, change the story."
Chelsea host Madrid next Tuesday, with the winners facing Manchester City or Bayern Munich in the semi-finals.
