Notable changes from Real saw Raphael Varane and Nacho back from injury and Carlos Casemiro dropped in the middle of the park.
The first bit of danger followed the host early on after a cutback from Alvaro Negredo to Jairo Izquierdo led him to cut inside and bring goalkeeper Thibaut Courtois to a save.
Madrid broke the deadlock on the half-hour mark when Karim Benzema finished off from the penalty spot after Vinicius Junior went down from a challenge in the area by Isaac Carcelen.
Los Blancos made it two goals three minutes later after Alvaro Odriozola met up at the back-post to head home a smart delivery from Benzema.
Casemiro could've made it three for the Spanish giants four minutes later from a close-range attempt off a knockdown by Benzema, but the Brazilian dragged over the top.
The visiting side did indeed add a third goal five minutes before halftime when Benzema grabbed a brace with a close-range header beyond the reach of goalkeeper Jeremias Ledesma.
Cadiz made a massive four changes at the break, as Ivan Saponjic, Anthony Lozano, Cala and Carlos Akapo came on from the substitutes bench.
Los Blancos were a bit on the back-foot ten minutes within the restart after Cala looked to make an impact from a header off Izquierdo's corner-kick, but he went over the top.
The away side pushed for more goals towards full-time, but subsequently held ground for the victory and top spot at the same token.
