BILBAO, Spain --
Athletic Bilbao roared to a stunning 4-0 victory over Barcelona in the first leg of the Spanish Super Cup after striker Aritz Aduriz picked apart the European champions with a second-half hat trick on Friday.
It was Barcelona's biggest loss under coach Luis Enrique, whose decision to start several reserve players helped the Basque hosts focus on keeping Lionel Messi in check.
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Before Aduriz turned it into a rout, Mikel San Jose set the tone in the 13th minute by lobbing Marc-Andre ter Stegen's long clearance back over the goalkeeper's head with a shot from inside the center circle.
Barcelona will have to rebound on Monday at Camp Nou to keep alive its quest to repeat the six titles in one year won in former coach Pep Guardiola's first season.
''We know how difficult it is to beat Barcelona and scoring four goals is almost impossible,'' Aduriz said. ''But we cannot think four goals are enough against Barcelona at its stadium.''
Last May, Barcelona beat Bilbao in the final of the Copa del Rey with Messi scoring a spectacular opening goal before adding a second strike for a 3-1 win.
That Barcelona squad was unrecognizable on Friday night, and not just in its faces. The European champion lacked both its usual verve in attack and discipline at the back.
Luis Enrique shuffled starting 11 from Tuesday's European Super Cup, when Barcelona's players tired and squandered a 4-1 lead before Pedro Rodriguez netted in added time for a 5-4 victory over Sevilla.
That gave it four straight titles to go with the Champions League, Spanish league and Copa del Rey from last season.
Pedro started at San Mames for Neymar, out with the mumps, while Sergio Roberto played for Andres Iniesta and little-used Thomas Vermaelen and Marc Bartra anchored the defense.
''We lost, but nothing is over yet,'' Barcelona midfielder Javier Mascherano said. ''We still have a game at home, and we have won many games by a big score, although it will be difficult.''
Bilbao's intense pressure quickly shattered the cool control that Barcelona wanted to impose, leading to San Jose's great goal.
Ter Stegen left his area to head out a long ball, but the German `keeper could only watch as San Jose controlled his long clearance and sent the ball sailing back into the net.
Messi didn't muster Barcelona's first shot on goal until first-half injury time when goalkeeper Gorka Iraizoz did well to palm his free kick over the bar.
Barcelona briefly pushed for an equalizer after the restart, as Pedro poached a poor clearance and blasted the ball off the crossbar in the 50th. Messi then followed moments later with a low shot that was parried by Iraizoz.
But seconds after Iniesta came on for Rafina, youngster Sabin Merino dribbled past Dani Alves and placed a cross for striker Aduriz to rise over Mascherano and head the ball home in the 53rd minute.
The story repeated itself 10 minutes later, when Luis Enrique ditched his experiment and sent on Ivan Rakitic for Roberto, only for Aduriz to score after Barcelona's defense again failed to clear the ball in the 62nd.
Aduriz brought up his hat trick from the spot six minutes later, after a senseless penalty committed by Dani Alves, who pushed Xabier Etxeita over during a corner kick.