France striker Benzema is profiting from the absence through injury of first-choice centre forward Gonzalo Higuain and followed up on his Champions League triple against Auxerre on December 8 with another impressive performance.
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He opened the scoring against Levante in the sixth minute at the Bernabeu when he robbed Miquel Robuste, bamboozled Xavi Torres with a couple of stepovers and lashed a low shot past Gustavo Munua.
Ronaldo set up Mesut Ozil for the home side's second in the 10th minute and Benzema made the most of another defensive error to make it 3-0 in the 31st.
The former Olympique Lyon forward, who has struggled to settle in Spain, bagged an assist moments before halftime when he played in Ronaldo for Real's fourth.
Tempers flared briefly at the start of the second half. Benzema clashed with Robuste off the ball and ended up with a bloodied lip. Both players were booked by the referee.
The Frenchman, who turned 23 on Sunday, completed his hat-trick in the 69th and Ronaldo added two more in the 72nd and 74th as Levante, who held Real to a 0-0 draw in La Liga in September, were overwhelmed.
Substitute Pedro Leon added the eighth in the 90th.
FOOTBALL DEMONSTRATION
"It was good because I think the people want a demonstration of football and today we gave them one," Benzema said in an interview with Spanish television.
"I am going to work harder for the team and everyone," he added. "I am improving day by day and match by match."
Real have not won Spain's domestic Cup since 1993 and face a possible quarter-final against city rivals and 2010 runners-up Atletico, who won their first leg at home to Espanyol 1-0 earlier on Wednesday.
The second legs are scheduled for January 5.
In other first legs on Wednesday, holders Sevilla twice came from a goal down in a 5-3 win at home to Malaga and Real Mallorca lost 4-3 at Almeria.
On Tuesday, Athletic Bilbao held a weakened Barcelona to a 0-0 draw at the Nou Camp, while Valencia were unable to convert their domination of local rivals Villarreal into goals and were also held 0-0.
Second division Cordoba came from a goal down to draw 1-1 against Deportivo La Coruna.
