Bruno Fernandes booked Manchester United's place in the Europa League semi-final from the penalty spot as the Red Devils ran out 1-0 winners over Copenhagen.
Ole Gunnar Solskjaer's men had just one leg to get the job done in the quarter-finals in Cologne, and needed extra time to see off their Danish opposition.
Both Mason Greenwood and Marcus Rashford thought they'd opened the scoring for United in either half after finding the back of the net.
But the goals were ruled out for being offside, as United struggled to make a breakthrough.
Eventually the Red Devils nosed in front in extra time through Fernandes, with the Portuguese finally beating Copenhagen keeper Karl-Johan Johnsson, who was unbeatable until that point.
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Manchester United will now play either one of Sevilla or Wolves, who contest their Europa League quarter-final on Tuesday.
Here's five talking points from Cologne.
1. Red Devils looking shaky at the back
Solskjaer has done a great job of shoring things up at the back, with Harry Maguire and Aaron Wan-Bissaka fitting in seamlessly since signing for the Norwegian.
Manchester United conceded just 36 goals in the Premier League last season - the third-best in the English top-flight.
But on Monday - particularly early on - United were distinctly off colour defensively.
Copenhagen consistently hit the Red Devils on the break, putting United under extreme pressure on more than one occasion, and very nearly scoring.
United eventually made up for their sluggish start, and increasingly moved play up the pitch.
But United's performance will give Solskjaer some food for thought, as the Red Devils can't afford to go behind in these intense one-leg knock out matches.
2. United struggle against a low block
Copenhagen boss Stale Solbakken is good friends with Solskjaer, and it seems he's very aware of how he sets up his Manchester United side.
On Monday, the Norwegian deployed the perfect low block against Solskjaer's Red Devils, cutting off all space for them to play, and leaving United extremely frustrated.
Ahead of the match Solbakken played down his side's chances.
"We need a bit of luck. We maybe need a United that doesn't fire on all cylinders," he said.
"We need to be perfect defensively. We need to take that one or two or three chances we get during a game like this."
And the mind games perhaps got to United, with the Old Trafford giants seemingly underestimating their opponents' ability to make the game difficult.
3. Greenwood always hits the target
These days the focus is on inside forwards and false nines.
Old fashioned number nines are seemingly a thing of the past, as players like Jadon Sancho, Marcus Rashford, Mohamed Salah and Sadio Mane rule the roost.
It's because of this that Greenwood is something of throwback center-forward.
Unlike other strikers who needs four of five chances to score, Greenwood needs just one.
He's like Ruud van Nistelrooy before him - Teddy Sheringham, Jermaine Defoe similar - in that he is absolutely lethal in front of goal.
And, everyone around him knows that if he has the ball at his feet in or around the box, there's only one place it's ending up.
If it wasn't for the fact Greenwood mistimed his run on Monday, he would have opened the scoring from the tightest of angles in the first half in Cologne.
An extremely talented youngster with a very bright future.
4. Off the woodwork!
Manchester United have hit the woodwork eight times in the Europa League this season - that's more than any other club in the competition.
And on Monday it seemed like a breakthrough would never come with both Fernandes and Greenwood hitting the post for United.
To add insult to injury, the Red Devils also had two goals ruled out for being offside.
Eventually United would find a breakthrough from the penalty spot, but they needed extra time to carve out an opportunity.
A frustrating day all round with United not at the races, Copenhagen defending admirably, and the footballing gods seemingly against Solskjaer's side.
5. Fernandes from the penalty spot
It's a regular occurrence these days.
The Red Devils getting a spot-kick and the one and only Fernandes stepping up to stick the ball in the back of the net.
Fernandes hasn't missed a penalty in four seasons now, and made sure this Monday night wasn't the match in which he faltered.
The Red Devils were crying out for a moment of magic from a drab game in which Copenhagen overloaded their box to keep United at bay.
And it was Fernandes who provided it.
The Portuguese has now been directly involved in more goals in all competitions than any other player in the Premier League since he signed for United on January deadline day.
He has 11 goals - his last four from the penalty spot - and has made eight assists.