LaLiga champions FC Barcelona were stunned by Athletic Bilbao who grabbed a 1-0 win thanks to a late strike from veteran forward Aritz Aduriz.
Frenkie De Jong and Antoine Griezmann were handed their Barca debuts, with Sergio Busquets and captain Lionel Messi missing the season-opener
Barca were threatened on six minutes through Inaki Williams, who forced goalkeeper Marc Andre Ter Stegen to a solid one-handed block. Williams was in on goal again from the wide side 15 minutes in, but Ter Stegen was up to the task with a low save from five metres.
Against the run of play, Barca nearly profited from a sloppy back-pass on 31 minutes but Luis Suarez smacked against the post. However, that was the Uruguayan’s last piece of action as he went off through injury four minutes later with Rafinha his replacement.
Rafinha almost profited from a cut back on the wing three minutes to halftime, but Unai Simon tipped the Brazilian's effort onto the crossbar.
The Catalan giants clawed their way back in the contest, as Rafinha, Griezmann and Ousmane Dembele flooded the opposite area.
Athletic’s retreat allowed Barca to throw numbers forward which almost paid dividends on 70 minutes, however Ivan Rakitic ballooned over the top.
A solo entry into the area from Rafinha almost led to the winner on 79 minutes, but a defensive intervention denied the Brazilian once again.
From nowhere, Athletic pulled the cat out of the bag in the final minute with an acrobatic bicycle kick from substitute Aduriz.
In the final minutes, Barca pushed for an equaliser but Aduriz had done enough from the bench to stun their more illustrious opponents.