Bruno Fernandes' extra-time penalty edged Manchester United through to the Europa League semi-finals as Ole Gunnar Solskjaer's side toiled to victory against gutsy FC Copenhagen.
An overturned penalty, two disallowed goals and efforts off the woodwork frustrated the Old Trafford giants during a goalless 90 minutes in which the Danish side looked dangerous despite failing to register a shot on target.
Fernandes would save the day from the spot in extra time, securing a 1-0 victory and a last-four date against Wolves or Sevilla back at Stadion Koln this Sunday.
Solskjaer will need a vastly-improved performance if they are to get any further than that, though, as their tired, sloppy play was compounded by Copenhagen playing with the intensity one would expect from a side in their first-ever European quarter-final.
United struggled to find a rhythm in the stifling Cologne heat but had the better of the chances, with the VAR ruling out a penalty and then Mason Greenwood's emphatic strike in the first half.
Marcus Rashford saw a goal ruled out for offside before Fernandes hit the post in the second period, with Anthony Martial superbly denied as regulation time wound down.
The match was on a knife-edge as play headed into extra time, which was just five minutes old when Fernandes scored from a spot-kick won by Martial.
Copenhagen retained hope as goalkeeper Karl-Johan Johnsson produced a string of superb saves and Victor Lindelof became the latest to hit the goal frame, but there was to be no late leveller for Stale Solbakken's dogged side.