Madrid - European champions Barcelona were stunned by an Aritz Aduriz hat-trick and brilliant opener from Mikel San Jose as Athletic Bilbao took a 4-0 lead into the second-leg of the Spanish Super Cup.
Barca had picked up their fourth trophy of the year in Tbilisi on Tuesday with a 5-4 win over Sevilla in extra time in the UEFA Super Cup.
But those exertions clearly took their toll as an under-strength side was dominated from the off by the Basques.
San Jose fired in a volley from just inside the Barca half to break the deadlock before Aduriz struck three times in 15 second-half minutes to give Ernesto Valverde's side a huge advantage heading into Monday's second-leg.
Barca's defensive deficiencies had been laid bare in blowing a 4-1 lead against Sevilla that forced them into an energy-sapping extra 30 minutes in the Georgian capital.
However, with Gerard Pique among four first-team regulars rested the Catalans were even more of a shambles at the back and this time couldn't be rescued by an off-colour Luis Suarez and Lionel Messi.
It is the first time in 14 years that the Catalans have conceded four times in two consecutive games.
San Jose began an unforgettable night for the hosts in fitting fashion as he pounced on Marc-Andre Ter Stegen's headed clearance to fire the ball beyond the German 'keeper from just beyond the halfway line.
Barca slowly began to get back into the game, but they didn't seriously threaten until first-half stoppage time when Pedro Rodriguez was clipped on the edge of the area.
Messi scored twice from free-kicks against Sevilla in midweek and couldn't have struck this one much better as Gorka Iraizoz flew to his right to claw the ball away from the top corner with an outstretched hand.
The visitors twice came close to an equaliser at the start of the second period as Pedro fired off the underside of the bar before Iraizoz made another brilliant save to deny Messi from Suarez's clever cut-back.
However, the game turned back in Athletic's favour when Sabin Merino beat Dani Alves for pace and his cross was powerfully headed home by Aduriz.
More poor defending from Luis Enrique's men was punished nine minutes later as the ball ricocheted around the area before Aduriz smashed home on the turn.
And the veteran striker was given the chance to complete his hat-trick when Alves was adjudged to have pulled down Borja Etxeita from a corner and Aduriz confidently sent Ter Stegen the wrong way from the spot.
Messi and Suarez tried to lift the visitors in the final quarter, but even they couldn't find a way past a resolute Athletic defence.
Barca now have a huge mountain to climb at the Camp Nou on Monday if they are to maintain their push to emulate Pep Guardiola's Barca side which won six trophies in a calendar year six years ago.