Champions Barcelona suffered a defeat on the opening day of the La Liga season after a sublime winner by Aritz Aduriz handed Athletic Bilbao victory.
Barca produced a lacklustre performance and were punished when substitute Aduriz, 38, found the net with a stunning 89th minute scissor-kick.
Luis Suarez and Rafinha both hit the woodwork for Barca but the visitors struggled to impress despite fielding new £107m signing Antoine Griezmann.
Suarez was forced off with an injury.
With Lionel Messi missing with a calf injury and Philippe Coutinho not involved amid reports he is set to join Bayern Munich on loan, Barca suffered a further blow as Suarez hobbled off before half-time.
Ernesto Valverde's side - chasing a third successive La Liga title - also included midfielder Frenkie de Jong, a £65m signing from Ajax but were disjointed for long spells and managed just two shots on target.
Former Spain forward Aduriz has already announced he is retiring at the end of the season after a career spanning 20 years.
He threw himself into the air and acrobatically volleyed past Marc-Andre ter Stegen to earn Athletic Bilbao a first league win over Barcelona since 2013.