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 La Liga.
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.
Champions Barcelona welcome Real Betis later Saturday.