Watch Atleti and Barca in LaLiga LIVE on beIN SPORTS | Sign up to take advantage of our 7-day FREE TRIAL here | AU | NZ

Barca dominated for long periods in the first leg of their quarter-final tie at Camp Nou, which represented the teams' fifth meeting of the season and their second in four days, after the Blaugrana's 2-1 away win in LaLiga. 

The game turned on Pau Cubarsi being sent off for a last-man foul at the end of the first half, with Alvarez  scoring from the resulting free-kick. Hansi Flick's side continued to press even while a man down, but Sorloth's left-footed finish means Barca has a mountain to climb if it is to avoid a shock exit.  

Both sides had early opportunities as Juan Musso blocked Marcus Rashford's goal-bound effort with his legs, before former Barca forward Antoine Greizmann was inches away from getting on the end of Ademola Lookman's cross-shot.

Rashford had the ball in the net in the 18th minute, only for the goal to be disallowed for an offside against Lamine Yamal earlier in the move, as Barca started to turn the screw.

But when Atleti came forward against the run of play in the final moments of the half, it resulted in a huge double blow for Barca. 

A VAR review saw Cubarsi sent off after he clipped Giuliano Simeone from behind, denying him a one-v-one with Joan Garcia. From the resulting free-kick 25 yards out, Alvarez whipped a sensational effort over the wall and beyond the dive of Barca's goalkeeper.

Flick reacted by withdrawing Robert Lewandowski and Pedri at the interval, but Barca continued to look dangerous with Rashford their main threat. 

He hit the side netting after being forced wide while rounding Musso, then saw a blistering free-kick – from almost an identical position to Alvarez's goal – tipped onto the crossbar. 

Atleti was struggling to get out of its territory despite being a man up, but it silenced Camp Nou again in the 70th minute, as substitute Sorloth swept Matteo Ruggeri's first-time delivery into the far corner. 

Joao Cancelo hit the outside of the post as Barca continued to throw men forward, and they were applauded off at full-time despite their defeat, though they have plenty of work to do in the second leg next week.

After Sunday's 2-1 league defeat to Barca, Atleti boss Diego Simeone was furious that Gerard Martin saw a red card – for a studs-up challenge on Thiago Almada – rescinded. 

But another young Barca defender was less fortunate here, as Cubarsi became the first player in Champions League history to be sent off twice as a teenager.