Marcos Alonso's attention had been diverted for only a second and Lionel Messi was through.
It was not in the last minute, when tired legs and minds are most vulnerable to lapses in concentration, but the third, when Chelsea's rearguard should have been at its sharpest.
Alonso was hardly to blame. The ball ricocheted off him in the penalty area to Luis Suarez and momentarily the full-back's gaze followed to the Uruguayan's foot.
Behind, Messi scampered into space, controlled and drove under the body of Thibaut Courtois. Nine of Chelsea's 11 players were yet to even touch the ball.
There were spells at the Camp Nou on Wednesday when Messi seemed to be playing within himself. In the first half, a few minutes passed when he even locked into an area on the right wing, receiving possession, keeping it, winning a free-kick, taking a throw-in.
The ball was switched to the other flank but came back and Messi had hardly moved. It was as if he had been set a challenge to dictate the match from as small a space as possible.
Chelsea had chances, and Messi admitted afterwards Barcelona's place in the Champions League quarter-finals had not felt safe, until he slammed in a third for 3-0, 4-1 on aggregate.
"It was a really tough game," Messi said. "But we played strong as a team. Scoring so early meant we could control the game, but we didn't feel we had finished the tie until we got our third goal. Until then, we had to fight hard and suffer to achieve a very important qualification for the team."
Messi's second of the night was his 100th in the Champions League, coming after 123 games, 14 fewer than it took Cristiano Ronaldo to reach the same mark. He now has 541 goals in 625 Barcelona appearances and 37 in 46 this season.
"I never thought I would see a better player than Diego Maradona," former Barcelona striker Gary Lineker said on BT Sport. "He's the best dribbler I've ever seen, the best passer I've ever seen and he scores more goals than anyone I've ever seen."
- Argentina's 'chance to dream' -
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But with Messi, there is a need to look at what is left as well as what has already been and the 30-year-old might consider this season a prime opportunity to bolster what is his, relatively, light Champions League haul.
Since his first in 2006, Messi has clinched three more, but only one since 2011. His total of four Champions League triumphs puts him on a par with the likes of Ronaldo, Gerard Pique, Andres Iniesta, Xavi, Samuel Eto'o and Clarence Seedorf. No player has yet managed to win five.
If the current era is to be remembered in the shade of blue and red stripes, Barcelona and Messi might therefore feel there is work still to be done. Barca own a meagre five European crowns to Real's 12, with the Catalans' deficit extended in recent years rather than cut.
And then there is the World Cup, Messi's holy grail. Four years ago in Brazil, he was named player of the tournament but in the final, against Germany, he flashed wide of the far post with only Manuel Neuer to beat and the score goalless in the second half. Germany went on to win 1-0.
Messi will want to put that right in Russia, where he can collect the last major honour to have eluded him and silence those that maintain his career will always carry an asterisk until he does.
"We are delighted that Messi's form gives us the chance to dream," Argentina coach Jorge Sampaoli said on Thursday, ahead of upcoming friendlies against Italy and Spain. "With the great patch that Messi is going through, with the World Cup coming soon, the dream is to take advantage of it and for the team to achieve a connection with him."
Messi will be 31 this summer, 35 by the time the next World Cup concludes in Qatar, and no-one would bet against him being there too. But these next few months would appear to offer a rare chance to break new ground.
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