A match where the odds were heavily stacked against the Rojiblancos for a number of reasons, not least the fact that they'd not won at Camp Nou since 2006, were on a three-match losing run away from the Metropolitano, and had only won once away in the UCL this season.
Barca come flying out of the blocks
Barcelona had also won eight of their last nine games in all competitions (and were unbeaten at home in 17), with the last time they'd tasted defeat being the galling 4-0 loss against Atleti in the Copa del Rey semi-final first leg.
With the final of that competition coming a few days after the second leg of the UCL tie, it will be interesting to see how Diego Simeone sets the mattress makers up next Tuesday.
The Argentine certainly had a game plan at Camp Nou, allowing Barca to seize the initiative from the get-go. Marcus Rashford squandered two chances in the opening three minutes and would later go on to miss a handful more.
Julian Alvarez went close directly after as if to signal Atleti's intent. However, the pattern of the game quickly developed into attack against defence after that, with Barcelona setting the pace.
Lamine's passing was poor
It was clear enough from the early exchanges that the Catalans were intent on giving Lamine Yamal as much responsibility as he could handle, though it's fair to say that the 18-year-old prodigy didn't live up to the hype in such a big game.
His passing was off all night, and consistently not finding his teammates led to a low pass completion of 77.4%. Only Rashford had a worse output in this regard, and with both wide players therefore ineffective in that sense, it was always going to be difficult to create chances.
The balance of play could be seen by just how busy Atleti's defenders were being kept, and Nahuel Molina deserves a lot of credit for winning all four of his tackles and winning back possession on eight separate occasions, the most for his side on the night.
Barca's frustration at Atleti's tactics was clear enough, but with Giuliano Simeone and Alvarez involving themselves in 30 one-on-ones alone, they were just working harder than the hosts at closing down space and not allowing Barcelona's players time on the ball.
Cubarsi's red card changed the game
By the half-hour mark, only 9.1% of play had been in Barca's defensive third of the pitch, allowing the likes of Joao Cancelo to bomb forward at every opportunity. Indeed, Barca's collective 69.1% possession in the first half evidenced just how much control the hosts had.
The Portuguese's five touches in Atleti's penalty area were only surpassed by Rashford and Lamine, the former having a goal disallowed after the latter was flagged for offside as the game headed into the break.
Cancelo's raiding had to be curtailed in the second half as not long after Barca's disallowed effort, Pau Cubarsi's clumsy challenge on Simeone saw him red-carded after a VAR review.
From the resulting free-kick, Alvarez bent a delicious delivery beyond the grasp of Joan Garcia, and Atleti led with only their second shot on target (both from the Argentine).
Cubarsi's second red in the Champions League, in what was his 26th appearance in the competition, made him the youngest player since Benfica's Antonio Silva to have been sent off on more than one occasion in the UCL.
An 11th first-half goal conceded in 11 UCL games for the Catalans means that only Qarabag (15) have conceded more in this season's competition, which is perhaps the one stat that Barca may dwell on if they do not qualify for the semi-finals.
Pedri and Lewy off at the break
Playing a high defensive line is all well and good if it's working well, but if it's consistently beaten in the bigger games, maybe a rethink is needed.
Taking off Pedri and Robert Lewandowski at half-time was a huge call from Hansi Flick, though the pattern of the game didn't change, even with 10 vs 11.
Juan Musso's seven saves for Atleti contrasted sharply with just the one for Joan Garcia, and just the one tackle made in the game from Barca's entire starting back four also tells its own story.
For all their attacking verve and spirit, the hosts still couldn't penetrate a brilliantly organised Rojiblanco defence.
Sorloth with a simple finish
By the hour mark, Rashford had taken seven shots, four of which were on target, and yet he was unable to find a way past Musso.
The second goal of the game, when it arrived in the 70th minute, was simplicity itself. Get the ball up to Alexander Sorloth and let him do the rest.
No passing for passing's sake, or giving Lamine the licence to dribble on 19 separate occasions for them all to come to nothing.
Just direct and penetrative football, and it was game over for the Catalans at that point.
The tie itself still isn't over, of course, and should Barca get an early goal in the second leg, it could set things up for a very intriguing 90 minutes indeed.
However, they'll need to be much more incisive if they want that to become a reality.
