The La Liga leaders were on top in the all-Spanish tie at Camp Nou before Pau Cubarsi was sent off for bringing down Atletico's Giuliano Simeone, who was through on goal.

Alvarez whipped home the free-kick and Sorloth doubled their advantage with 20 minutes remaining to give Atletico a strong chance of reaching the semi-finals.

Diego Simeone's side, who have never won the competition, reached the 2014 and 2016 finals but were beaten by rivals Real Madrid on both occasions.

Barca, semi-finalists last season, need to make a huge comeback next Tuesday in Madrid if they are to stand a chance of lifting the trophy they last won in 2015.

Atletico knocked Barca out of the Copa del Rey in the semi-finals in March and this was the second of three meetings between the sides in a space of a fortnight.

Barcelona started well with Marcus Rashford, on loan from Manchester United, their main threat down the left.

On the right flank Atletico tried to suffocate teenage star Lamine Yamal with defensive numbers, allowing Rashford more room.

The first chances fell to the England international, who scored against Atletico in La Liga on Saturday, but Musso denied him.

At the other end Alvarez, who has been linked with a switch to Barcelona in recent months, tested Joan Garcia.

The busy Rashford, constantly escaping beyond Nahuel Molina, volleyed narrowly wide and then rolled a shot into the bottom corner but it was disallowed for offside against Yamal in the build-up.

Even though the 18-year-old was up against three Atletico defenders he produced some superb skill to break free of their shackles, but Robin Le Normand blocked his shot.

Veteran Atletico midfielder Koke was fortunate to stay on the pitch after fouls on Dani Olmo, Pedri and Yamal were only punished with a yellow card, to Barca supporters' fury.

Hansi Flick's side were controlling the game with Atletico talisman Antoine Griezmann quiet against his former team, but a moment of clumsiness from Cubarsi dramatically gave the visitors the upper hand before half-time.

The 19-year-old centre-back, chasing Giuliano Simeone as he ran through on goal to reach a ball in behind Barca's defence from Alvarez, clattered the Atletico coach's son from behind.

Although referee Istvan Kovacs only produced a yellow card for Cubarsi, he upgraded it to a red after a VAR review.

Worse was to come for Barca, with former Manchester City striker Alvarez whipping the free-kick into the top corner.

- Sorloth doubles down -

Barca coach Flick took action at half-time, withdrawing the quiet Robert Lewandowski and the booked Pedri for energetic midfield tandem Gavi and Fermin Lopez.

It helped the hosts continue to battle well with 10 men, as Barca kept generating chances for Rashford. Yamal sent the forward through on goal but after rounding Musso could only hit the side netting.

Le Normand brought down Yamal on the edge of the box and Rashford's free-kick was tipped over by Musso.

It was against the run of play again that Atletico snatched their second.

Griezmann and Matteo Ruggieri combined and the latter crossed for target man Sorloth, who outmuscled Gerard Martin and fired past the helpless Garcia.

Musso later saved from Joao Cancelo and Yamal, but Barca could not pull a goal back, leaving the five-time winners in a tough spot ahead of the second leg next Tuesday.