Markel Susaeta scored decisively as Athletic Bilbao claimed a deserved 1-0 friendly win over Borussia Dortmund at AFG Arena.
Thomas Tuchel named close-season recruits Mario Gotze and Andre Schurrle in a strong line-up but his team faltered during the middle of the first half in St Gallen.
Star forward Pierre-Emerick Aubameyang had a golden opportunity to equalise three minutes before the interval but saw his penalty saved.
Teenage sensation Christian Pulisic made a lively start for Dortmund and sliced wide when Aubameyang's cutback was partially cleared, but Tuchel's men fell behind after 20 minutes.
Dortmund's claims for a foul by Aritz Aduriz on Julian Weigl fell on deaf ears and, to compound their frustration, Aduriz played the final pass of the resulting slick Bilbao move for Susaeta to slot home.
The goal unsettled Dortmund considerably and Athletic winger Inaki Williams had a close-range effort deflected over before whipping a shot wide of the far post.
Javier Eraso played Aduriz through for the veteran striker to draw a fine save from Roman Weidenfeller before the Dortmund keeper's opposite number kept out Aubameyang from 12 yards.
Mikel San Jose was harshly penalised when Gotze's cross ricocheted up on to his arm, but Aubameyang did not strike cleanly and Gorka Iraizoz plunged to his right to save one-handed.
Iraizoz clawed away a swerving Aubameyang attempt from long range before Pulisic hit the post to conclude an eventful first half.
The anticipated raft of substitutions - Aubameyang, Gotze and Andre Schurrle made way among eight Dortmund alterations at the interval - hindered the game as a spectacle.
Athletic continued to look the most likely, with Williams again going close to the goal his endeavours deserved and boss Ernesto Valverde had plenty of cause for encouragement with his side's La Liga opener at Sporting Gijon 12 days away.