Atletico coach Diego Simeone said his team put in their "weakest" performance of his 12-year reign in a 3-0 thrashing by Valencia on Saturday in LaLiga.
Los Che have started the season well after flirting with relegation until the final day of the previous campaign, and this was their best performance yet under coach Ruben Baraja.
Simeone has faced many different Valencia coaches over almost 12 years in charge and had only lost twice before this defeat at Mestalla in 28 clashes.
"We had the weakest game since I arrived at the club, and we have to congratulate our opponents," said Simeone.
Defeat was the worst way for Atletico to prepare for the start of their Champions League campaign on Tuesday at Lazio.
"They came into the game with a lot more intensity and if you don't match it ... it's impossible to win any game," said Atletico goalkeeper Jan Oblak.
"They deserved to win, we have to congratulate them and we have to look in the mirror. We know we can give a lot more than we gave today.
"We lacked everything today... we played a very bad game, we lost 3-0 and that hurts a lot."
Atletico suffered their first defeat of the season as they went down without a fight.
Spanish striker Duro, 23, slid in at the near post to fire into the roof of Oblak's net after just five minutes.
The forward kept his composure to beat the Slovenian goalkeeper again when sent through on goal by Fran Perez after shrugging off Axel Witsel.
"It was important to win after two defeats and we did it against a great opponent," said Duro.
"We may be a small squad but we will give everything."
Javi Guerra struck the third goal with a fine solo move and finish inside Oblak's near post as Los Che continued to dominate.
Valencia could have increased their lead but Oblak saved well from Diego Lopez in the final stages, among other openings.
"I am not at all angry, I know the players we have, the group in the dressing room, but I accept it when we've played a bad match," added Simeone.
Earlier Athletic Bilbao earned a convincing 3-0 win over Cadiz to climb provisionally second behind leaders Real Madrid, who host Real Sociedad Sunday.
FELIX AND CANCELO STRIKE AS BARCELONA SHRED REAL BETIS
New arrivals Joao Felix and Joao Cancelo struck their first goals for Barcelona in an impressive 5-0 rout of Real Betis on Saturday in LaLiga.
Portugal forward Felix fired Barcelona ahead, with further goals from Robert Lewandowski, Ferran Torres and Raphinha sinking the visitors, before Cancelo polished off the rout with a fine individual effort.
Barcelona's emphatic victory places them provisionally top of the table, a point clear of rivals Real Madrid who host Real Sociedad on Sunday.
Barcelona played with a vibrancy they lacked at times last season, even as they lifted LaLiga, warming up for their Champions League opener on Tuesday against Royal Antwerp.
Felix was at the core of it with an impressive display after Xavi handed him and compatriot Joao Cancelo their first starts for the club following loan moves on deadline day and both quickly impressed the home fans.
Betis, lining up with a back four composed only of former Barcelona players, played with attacking intent, unlike many other teams who will visit the temporary Olympic Stadium home of the Spanish champions.
Luiz Henrique sent a rasping effort across the face of goal and Ayoze Perez blasted into the side netting as Manuel Pellegrini's side started well.
However three of Barcelona's summer signings combined to put the hosts ahead after 25 minutes.
Cancelo won the ball back with a thumping tackle and Oriol Romeo dropped a cross into the area for Felix.
The on-loan Atletico forward controlled the ball, knocked it past goalkeeper Rui Silva and then squeezed home clinically from a tight angle.
Lewandowski doubled Barcelona's lead when he ran onto Andreas Christensen's pass and slotted home, but the goal was crafted by Felix, whose clever dummy allowed the ball to find the Polish striker.
Marc-Andre ter Stegen produced a superb save to deny Willian Jose after Isco cleverly found the Brazilian forward near the penalty spot.
Silva foiled Felix just before halftime at the end of a quick counter-attack, as Barcelona sought to put the game out of sight.
Torres, selected ahead of teen starlet Lamine Yamal, netted the third, with a low free kick around the wall – the first Barcelona have scored since a Lionel Messi strike in May 2021.
Raphinha, who replaced Torres, quickly added the fourth just moments after entering the fray with a drilled effort from outside the area into the bottom corner.
On-loan Manchester City defender Cancelo gleefully added the fifth to cap an excellent individual display, skinning former Barcelona left-back Juan Miranda and driving beyond the despairing Silva.
Whether new-look Barcelona can match Europe's best remains to be seen but this rampant display highlighted the improved attacking quality at their disposal this season.