Raphael Varane and Lucas Vazquez were on target in the first half of the last-16 clash at La Romareda as Madrid barely had to move out of second gear.
9 - @realmadriden's Vinícius Júnior has been involved in more goals than any other @LaLigaEN player in the Copa del Rey since the start of last season (three goals and six assists). Prince. pic.twitter.com/NfCZeWostU— OptaJose (@OptaJose) January 29, 2020
Tougher tests are on the horizon for LaLiga's leaders, starting with El Derbi against Atletico Madrid on Saturday, but it was a job well done for Zinedine Zidane's side.
Watch Real Madrid vs. Atletico Madrid, Saturday February 1 @ 10:00AM ET on beIN SPORTS USA
Madrid caught Zaragoza napping after six minutes when Toni Kroos' low ball across the box went through Vinicius' legs and was smashed in by Varane.
The visitors were playing as though the game were a mere training exercise, although Alphonse Areola had to be sharp to tip Shinji Kagawa's deflected shot over.
Miguel Linares' questionable decision to dribble out of his own area was punished as Kroos pick-pocketed the forward and laid on for Vazquez to brilliantly smash home Madrid's second.
Kagawa scooped an effort over in a rare moment of note in the second half before Madrid put the game beyond doubt.
Vinicius played a neat one-two with James Rodriguez and clipped his effort past Alvaro Raton into the bottom-right corner.
Areola kept out Luis Suarez's one-on-one effort, but Madrid had a fourth 11 minutes from time when Benzema tapped in from Dani Carvajal's teasing low centre at the end of a well-worked move.
What does it mean? Madrid tune up nicely for the derby
Zaragoza are riding high in the second tier but were no match for Madrid, who are now unbeaten in 20 matches across all competitions. Zidane will be happy his side did not have to shift through the gears ahead of Saturday's derby with Atletico at the Santiago Bernabeu.
Madrid in Kroos control
Kroos was typically influential in the heart of Madrid's midfield and finished with two assists. The Germany international dictated the tempo from the centre of the park.
Lax Linares punished
Zaragoza were mainly chasing shadows but had a couple of moments, particularly through Kagawa, to threaten Madrid. Linares' ill-judged attempt to jink out of defence proved costly, though, as Madrid quickly made it 2-0 to leave the hosts with a mountain to climb and he was hauled off just before the hour.
Madrid face the tricky task of a derby with Atletico in LaLiga on Saturday, while Zaragoza visit Cadiz a day later.