Atlético set the tone for an incredible first half early, with Giuliano Simeone seeing his shot brilliantly saved by Joan García when clean through.
Just six minutes in, the normally reliable García took his eye off a back pass from namesake Eric García, and let the ball trickle over the line to hand Atlético the lead.
Despite the visitors dominating possession in the early stages, a well-worked move from the hosts saw Antoine Griezmann in acres of space, and his low left-footed finish had Diego Simeone’s men two goals to the good by the 15-minute mark.
Barça then hit the woodwork through Fermín López, before Julián Alvarez could only despair as Jules Koundé cleared his shot off the line, in what was already a breathless encounter.
Time and again, Atleti pushed forward and had multiple chances to go further ahead, before they eventually did as Ademola Lookman fired home at the end of an incisive counter.
Unbelievably, another break down Barça’s right-hand side in stoppage time saw Alvarez eventually picked out, and he smashed home from the edge of the area for his first goal since early December, marking just the second time since 2004/05 that Barcelona had conceded four goals before half time.
Juan Musso had to be on his toes straight after the break as Fermín got a shot away on target, and after an astonishing eight-minute VAR delay, Pau Cubarsí’s goal was ruled out for offside.
That appeared to knock the stuffing out of the visitors, whose passing became increasingly wayward. To compound Barça’s misery, Eric García was sent off late on for a professional foul.
Atleti, not unexpectedly, took their foot off the accelerator in the latter stages of a historic night that will live long in the memory and marked their first win in nine Copa del Rey clashes with Barcelona (L6, D2).
It was also Hansi Flick’s first defeat in the competition, and puts a huge dent in the Catalans’ aspirations of a 33rd title.
Flashscore Man of the Match: Giuliano Simeone (Atlético Madrid)
