It took Atlético Madrid just 10 minutes to find Barcelona's weak spot. The left side of the Catalan defence, where João Cancelo lined up from the off, became the main outlet for Diego Simeone's attacks.
According to the newspaper Sport, three consecutive raids by Los Rojiblancos laid bare the Portuguese full-back's flaws. He struggled above all to judge when to push up and press, and when to hold his defensive line.
The first warning arrived through Julián Álvarez, who found himself one-on-one with a defender after Atlético exploited the space down Barcelona's left.
Cancelo failed to deal with it. He let the Argentine forward carve out a dangerous opening.
Back came the second attempt from the very same zone. Atlético kept hammering that flank and won a corner that nearly brought the opening goal.
Marc Pubill rattled a shot off the post, proof that the Madrid side had found an area to hurt Hansi Flick's team.
Kang-in Lee was the root of Barcelona's troubles, with Cancelo left to shoulder the heaviest burden.
The third attempt went unpunished no longer. Cancelo again got caught out, badly misjudging the moment to step towards his man. Álvarez seized on it and slid the ball to Giuliano Simeone, who finished off the move to put Atlético Madrid ahead.
Three gilt-edged chances down the same flank in just 10 minutes. The problem ran deeper than individual errors. A breakdown in defensive coordination gave Atlético something to attack from the first whistle.
The most telling moment came straight after the goal. Goalkeeper Joan García marched over to Cancelo, grabbed hold of him and started spelling out his mistakes.
Barcelona's goalkeeper tried to fix the Portuguese's positioning on the pitch, well aware that Atlético had found it far too easy to attack down that side.
That exchange between the players showed just how worried the start of the match had made them. Cancelo, blessed with enormous attacking quality, laboured through his defensive duties against an opponent that had drilled its attacks to target that exact area.
A fourth error in the first half could have brought Atlético's second. Under pressure from the Madrid side, Cancelo lost the ball in his defensive third. The move again ended with a shot from Julián Álvarez, which Joan García blocked between his legs, though not without clear difficulty.