The Portugal international put through his own net midway through the first half to consign the Liga champions to their fifth defeat of the season
In trying to head clear a corner from Nolito, the Portugal international inadvertently deflected the ball past a stunned Diego Lopez. The reigning Liga champions, led by a determined Ronaldo, tried desperately to rally, but substitute Karim Benzema squandered a gilt-edged opportunity to snatch a point in the dying seconds and the hosts held on to claim a win which lifted them out of the bottom three.
Los Blancos had scored nine unanswered goals in their two previous Liga outings, against Valencia and Getafe, and another landslide victory was envisaged against a side languishing in the relegation zone and led by a new coach in Lucas Alcaraz, who had had just two full days to prepare his side for the visit of the champions.
However, Madrid looked sluggish right from the off, perhaps still feeling the effects of a draining Copa del Rey semi-final first-leg clash with Barcelona in midweek. Indeed, they did not fashion a chance worth of mention before falling behind in the most surprising of circumstances midway through the first half, Ronaldo heading a corner from Nolito past his own goalkeeper.
Desperate to make amends for the first own goal of his professional career, the Portuguese twice tried his luck from distance with 30-yard free kicks, but on both occasions, he was unable to test Tono in the Granada goal.
Yet those speculative long-range efforts were the best that Madrid could muster during an opening 45 minutes in which neither side managed to register a legitimate shot on target.
After such a tepid first-half display, Jose Mourinho unsurprisingly elected to make a couple of substitutions during the interval, sending on Benzema and Jose Callejon in place of Gonzalo Higuain and Sami Khedira.
The changes did not have any great impact, though, because while Madrid dominated possession, they continued to struggle to break down a Granada defence that was being wonderfully well marshalled by Diego Mainz.
Ronaldo did call Tono into action with a low drive from just outside the area that goalkeeper parried away for a corner, and repeated the feat with 19 minutes to go - but again to no avail.
Madrid really should have snatched a share of the spoils late on after Tono could only parry Callejon's shot into the path of Benzema. However, the Frenchman inexplicably fired wide from less than five yards out, even though there was only one Granada defender protecting the goal.
A late equaliser would have been desperately harsh on the hosts, though, who were full value for a shock victory which significantly boosted their bid to beat the drop, while at the same tame severely damaging Madrid’s hopes of overhauling city rivals Atletico Madrid in second place.