Athletic Bilbao thumped Atletico Madrid 3-0 in LaLiga on a miserable Saturday afternoon for Diego Simeone's side.
Nico Williams, Robert Navarro and Oihan Sancet's goals gave the Basque hosts a resounding victory at their San Mames stadium.
Elsewhere promoted side Deportivo La Coruna earned an exciting 3-2 win at Villarreal to climb provisionally fourth.
Athletic secured back-to-back wins under former Borussia Dortmund coach Edin Terzic, after losing their first two matches of the campaign.
Simeone's Atletico fell to their first defeat of the season, putting up little fight in the second half after Athletic made the breakthrough with two quick strikes from Williams and Navarro.
"Their two goals, in the blink of an eye, did us a lot of damage," Atletico's Pablo Barrios told Movistar.
Atletico must pick themselves up for Wednesday's visit to face Liverpool in the Champions League at Anfield.
"(We had) two minutes of inattention and poor decision-making in situations we should have handled better," said Simeone. "Bilbao capitalised on that lapse."
Striker Julian Alvarez, who tried to leave the club to join Barcelona this summer, returned as a substitute after illness in an ineffective second half appearance.
The visitors had the better of the opening period, although they failed to create many clear-cut chances after an early opportunity for Lee Kang-In.
The South Korean international struck a shot against the post in the third minute after the hosts were carved open.
Athletic goalkeeper Unai Simon saved from Alex Baena as Atletico threatened, while Sancet came close at the other end with a blast into the side-netting.
Pablo Barrios hit the post before the break for Atletico, who were sucker-punched early in the second half by Terzic's side.
The Williams brothers combined for Athletic's first, with Inaki's deflected shot headed home by younger sibling Nico after 46 minutes.
Navarro doubled Athletic's lead in the 48th minute, steering home after Inaki Williams played him in.
"We were good in the first half but the game got away from us in those (couple of) minutes," said Simeone.
"We lacked in the second half everything that we had in the first."
NO IMPACT FOR ALVAREZ
Simeone rang the changes, including sending on want-away striker Alvarez to little effect.
The Argentina international struggled to get involved in the game, and produced just one shot from the edge of the box, saved comfortably by Simon.
Sancet wrapped up Athletic's victory in the 90th minute when he stroked home a loose ball after a blocked cross.
"It's always good to win games, especially at home," said Terzic.
"Today we are happy and we are going to be very satisfied when we go to bed tonight, and then from tomorrow onwards we'll prepare everything to go for the next (game).
"We have six points from the last two games, but this won't help us to reach all our targets by the end of the season."
Pierre-Emerick Aubameyang scored for the fourth match running and set up two more as the former Arsenal and Barcelona striker excelled in Depor's surprise win at Villarreal.
Another ex-Arsenal attacker Nicolas Pepe put the hosts ahead but Luismi Cruz levelled for the Galicians before the interval.
Adama Traore's own goal gave Villarreal the advantage once more but Zakaria Eddahchouri and Aubameyang, 37, struck to claim Depor a superb victory.
Sergio Camello netted twice for Rayo Vallecano in an entertaining 3-2 win over Racing Santander.
On Sunday, LaLiga leaders Barcelona visit Valencia looking to move clear of Real Madrid, who lost 1-0 at Real Betis on Friday, among other fixtures.