It took Ronaldo just two minutes to extend his scoring streak to eight games as he headed home Gareth Bale's cross and Karim Benzema made the game safe when he doubled Madrid's lead just before half-time yesterday.
Bale teed up Ronaldo once more to make it 3-0 and the Portuguese then turned provider for Benzema to tap home his second of the game.
Ronaldo rounded off the scoring two minutes from time when he unintentionally deflected Pepe's effort home to take his tally for the season to 17 goals in just 11 matches in all competitions.
A fourth defeat in five league games for Athletic, meanwhile, means Ernesto Valverde's men end the weekend in the relegation zone.
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Madrid were back to full strength after coach Carlo Ancelotti had rested a few of his regular starters in the 2-1 win over Ludogorets Razgrad in midweek and took just two minutes to make the breakthrough against an Athletic side shorn of confidence.
Bale turned onto his weaker right foot, but still managed to pick out a perfect cross for the unmarked Ronaldo to head home at the back post.
However, the Basques settled midway through the half and Iker Muniain's spectacular dipping volley from the edge of the area forced Iker Casillas into action with a fingertip save.