Sergio Ramos' headed opener in the 15th minute set the tone for what was to come, with Karim Benzema adding a second nine minutes before half-time.
Gerardo Torrado squandered a glorious opportunity to reduce the deficit prior to the interval when he had a penalty saved by Iker Casillas and Gareth Bale punished that missed chance shortly after the teams emerged for the second period – heading home from a Cristiano Ronaldo cross.
With the result already beyond doubt, Isco wormed his way through the Cruz Azul defence to make it four 18 minutes from time.
Real will face the winner of Wednesday's clash between Auckland City and San Lorenzo in the final on Saturday.
Carlo Ancelotti named a strong starting XI to face the CONCACAF Champions League winners, and the biggest name on his teamsheet – Ronaldo – should have put Real ahead in the opening few minutes.
Benzema nodded down a right-wing cross, but the Portuguese attacker's toe poke from point-blank range was kept out by goalkeeper Jesus Corona.
Benzema found the side-netting with an 18-yard effort that whistled past the left-hand post as Real turned the screw, and the Spanish giants took the lead in the 15th minute – Ramos heading into an empty net after Corona had carelessly charged off his line in a bid to meet a Toni Kroos free-kick.
Joao Rojas caused alarm in the opposition penalty area shortly before the half-hour mark when he saw an effort deflect just wide off Ramos on a rare attack for the Mexicans.
Kroos, who won last year's tournament with Bayern Munich, forced a save with a well-placed shot from 20 yards soon after, and it was not long before Benzema doubled the Real advantage.
Dani Carvajal knocked the ball through the legs of defender Fausto Pinto before teeing up Benzema to prod home with a dinked cross from the byline.
Ramos was lucky to escape a red card when he was booked for bringing down Mariano Pavone in the penalty area, but captain Torrado was unable to beat Casillas from 12 yards – the Real goalkeeper diving to his right to pull off a second save from a spot-kick in as many matches.
Bale headed in from a Ronaldo cross five minutes into the second half for his third goal in three games, and the Welshman had the ball in the back of the net again soon after, although the flag had already been raised for offside.
Ronaldo appeared hungry to get in on the goalscoring action, and drew a save from Corona with an overhead kick before the hour mark, while another audacious effort was comfortably held by the increasingly busy keeper.
Pablo Barrera almost made an instant impact after coming on for Cruz Azul as his curling effort from just outside the area struck the left-hand post, but Isco's strike after a series of mesmerising Ronaldo step-overs compounded the Mexicans' misery.
Substitute Sami Khedira headed against the left-hand upright – missing out on a first goal since he found the net in Germany's 7-1 demolition of hosts Brazil at the World Cup earlier this year – and Ronaldo tested Corona with a free-kick in the closing stages, but it mattered little as Real comfortably booked their place in this weekend's final.
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