The fervent home fans were enraged when Athletic defender Daniel Vivian was sent off for pulling back Rasmus Hojlund and Fernandes slotted home the resulting penalty, after Casemiro had opened the scoring against the run of play in the semi-final first leg.

Fernandes rolled in a third before half-time as Ruben Amorim's side moved a step closer to the final, to be held at Athletic's San Mames stadium.

The hosts have been dreaming of winning a first European trophy on their own soil but their hopes were demolished by United's professional display in the north of Spain -- and Athletic's supporters argued, the refereeing.

Languishing in 14th in the Premier League, Champions League qualification for United is only possible with a Europa League triumph, as is access to the £100 million ($133 million) honeypot it entails.

Despite regularly crumbling under pressure this season, the Red Devils -- who produced a stunning comeback against Lyon in the quarter-finals -- first survived and then thrived in a hostile environment.

With May 1 a bank holiday in Spain the streets of Bilbao were filled with red-and-white striped shirts from the morning onwards, with thousands of fans turning up at their team's hotel to see the Athletic bus set off for the stadium.

The San Mames was rocking, with fans raising red and white cards around the stadium ahead of the game to welcome the players, all of them born or raised in the Basque country, as per the club's century-long policy.

"This is not the theatre of dreams, this is The Cathedral of football," it read on the back, a reference to the stadium's nickname.

Alejandro Garnacho's early strike gave the hosts a scare but the forward was offside.

Beyond that the early stages of the match for United were about gritting their teeth and holding off the Basque side and their raucous supporters.

- Red card rocks hosts -

Alex Berenguer forced a smart low save from Manchester United goalkeeper Andre Onana, and Inaki Williams headed narrowly over.

Victor Lindelof made a vital block to thwart Berenguer after Nico Williams fed his brother Inaki, who was given too much space on the right wing.

The hosts were ascendant and when former Real Madrid midfielder Casemiro opened the scoring for United it came as a shock.

Just as much of a surprise was the way United created the goal, with centre-back Harry Maguire dribbling down the right flank as Mikel Jaureguizar floundered in his wake.

The defender fizzed a cross into the area which Manuel Ugarte flicked on to the back host for Casemiro to nod home from close range.

It seemed like a smash-and-grab but soon United had a second, when Vivian was penalised for pulling back Hojlund as he tried to connect with a cross.

It was a key moment. The defender was sent off to add insult to injury, as Athletic fans howled in anger.

Fernandes rolled the penalty into the bottom right corner, sending Julen Agirrezabala the wrong way.

Athletic coach Ernesto Valverde made a double substitution, trying to stem the bleeding, but his team shipped a third before half-time.

Ugarte's clever backheel played Fernandes through on goal and he stroked home with ease.

Noussair Mazraoui crashed a shot off the crossbar from the edge of the box as United almost grabbed a fourth before the break.

Norwegian referee Espen Eskas and his colleagues were barracked by the home fans, seeing their aspirations to glory evaporate before their eyes.

They were further enraged, waving the white cards from the pre-match tifo to show their disgust, when Maroan Sannadi tumbled under pressure from Maguire as he ran towards goal but no foul was awarded.

United largely controlled the game in the second half, with Casemiro and Fernandes making life hard for the 10-man hosts, but they could not add a fourth despite probing.

The second leg takes place next Thursday at Old Trafford, ahead of the final on May 21 against Tottenham or Bodo/Glimt.