Manchester City headed into to the game sat top of their group with a perfect record.
The Sky Blues had won three in three, scoring 11 and conceding just one goal.
Copenhagen, meanwhile, were dead bottom of Group G with just one point. So, an easy City win was expected on Tuesday.
But, when all was said and done, that did not happen.
Manchester City not too surprisingly started the tie dominating the ball, with Julian Alvarez forcing a good save from Grabara six minutes in.
Man City thought they had broken the deadlock minutes later, with Rodri smashing in an effort from outside the box but it was ruled out for handball.
City were then handed a penalty for a soft handball, but Mahrez missed. Another VAR decision then saw Sergio Gomez sent off for denying a goalscoring opportunity.
Even after going down to 10 men, though, Man City continued to enjoy most of the possession. But they struggled to create much and were ultimately unable to find a goal.
Copenhagen player ratings
Kamil Grabara (6.5) – Made a good save six minutes in and stopped Mahrez’s penalty.
Lund Jensen (4) – Was poor in the air and won none of his ground duels.
Davit Khcholava (4) – Struggled to mark Julian Alvarez who was lively in and around the box.
Nicolai Boilesen (2) – Gave away a penalty in the 22nd minute.
Kristensen (5) – Made a few key passes and helped out at the back.
Stamenic (4) – Offered very little going forward.
Lukas Lerager (5) – Had a tough time marking KDB and Gundogan, but he did test Man City at the other end.
Kristiansen (4) – Passing was decent but he lacked any real creativity.
Viktro Claesson (3) – Did little of note despite City losing a man early on.
Arnar Haraldsson (5) – Was very isolated up top for much of the match but did help get Gomez sent off.
Mohammed Daramy (4) – Did little to test Joa Cancelo or Manuel Akanji.
Johannesson (5) - Had a great chance to score 73 minutes in but he could not reach the ball.
Kevin Dkis (N/A)
Manchester City player ratings
Ederson (5) – Did well with the little action he saw.
Joao Cancelo – Moved to left-back after the red.
Manuel Akanji (6) – Was moved to right-back following the red card to Gomez and did a solid job there.
Aymeric Laporte (5.5) – Good on the ball and was not tested defensively too much.
Sergio Gomez (1) –Was sent off 28 minutes in.
Rodri (6) – Superb strike early on but it was bizarrely ruled out for handball.
Kevin De Bruyne (6) – His creativity was hampered by Manchester City going down to 10 but he tested the Copenhagen keeper a few times.
Ilkay Gundogan (4.5) – Good in possession but did little of note.
Riyad Mahrez (1) – Missed a penalty in the first half and was taken off after 31 minutes due to the red card.
Jack Grealish (5) – Carried the ball forward well but struggled to link up with Alvarez.
Julian Alvarez (5) - Bright start that saw him force a good save from the Copenhagen keeper but his impact was limited after the red.
Ruben Dias (6) - Brought on following the red card and helped sure up City’s backline.
Bernardo Silva (3) - Was on for the final 15 to help City find a goal but failed to do so.
Phil Foden (3) - Did little of note.
Nathan Ake (N/A)