Beaten in only one of their last 26 LaLiga home games (W16, D9), the hosts flew out of the blocks, with Thibaut Courtois needing to make a sublime save to deny Nico Williams inside four minutes.
However, the most notable and controversial action of the opening exchanges would fall to the defending champions.
Referee Jose Maria Sanchez Martínez took centre stage, as he not only decided that Andoni Gorosabel’s challenge on Rodrygo in the area was a lawful one, but also that Kylian Mbappe’s effort within the same move should be ruled out for offside.
Athletic reacted well to that lifeline, and Alex Berenguer almost broke the deadlock shortly after the half-hour mark. Exquisitely put in front of goal by Inaki Williams’ backheel, the Spaniard somehow sent his effort high into the night sky with only Courtois to beat, leaving the hosts frustratingly still off the scoresheet in the first half.
The hosts didn’t falter from there, and they would finally find some joy only eight minutes into the second half.
Looking to atone for his earlier personal horror show, Berenguer did precisely that, as he slammed the ball home from underneath the crossbar after Nico Williams’ delicious delivery into the danger zone caused Courtois all sorts of problems.
Now chasing the game, Real Madrid spurned a glorious opportunity to get back on level terms from the penalty spot in the 68th minute, after Julen Agirrezabala punched Antonio Rudiger in the back of the head to stop him from getting to an incoming set piece.
Remarkably, Athletic’s deputising shot-stopper made immediate amends by correctly guessing the direction of Mbappe’s spot-kick.
Mbappe’s embarrassment of consecutive misses from 12 yards appeared to have been spared not long after, as Agirrezabala’s failure to hold onto the Frenchman’s effort from a distance allowed Jude Bellingham to fire home the rebound.
However, Federico Valverde’s loss of possession ultimately unravelled Real’s momentum, with Guruzeta capitalising on the untimely error by restoring Athletic’s lead with a sublimely-struck strike.
The overdue success over Real in LaLiga consolidates Ernesto Valverde’s side’s position in the top four, as the former Barcelona manager celebrates triumphing over his former team’s biggest rivals.