When Raheem Sterling got the game's fifth -- and City's third -- after 21 minutes, it was the fastest five goals had ever been scored in the Champions League, eclipsing the previous record of 24 minutes set by Dortmund and Legia Warsaw in 2016.
Sergio Aguero made it 4-2 on the night 15 minutes into the second half, putting City ahead in the tie for the very first time.
Just as it was looking like the hosts would put Spurs to the sword, a period of Tottenham pressure ended with substitute Fernando Llorente bundling home Kieran Trippier's corner to once again swing the tie in the visitor's favor.
With the game ticking into the final moments, Christian Eriksen's error allowed Aguero to cut the ball back to Sterling to score what appeared to be the winner. Tottenham's players were crestfallen, sat helplessly on the turf, as Pep Guardiola sprinted wildly down the touchline.
But seemingly from nowhere, the big screens inside the Etihad announced a VAR check was in progress for a potential offside against Aguero.
The entire stadium held its breath and when referee Cuneyt Cakir overturned the goal, it was as though the Etihad's energy had been deflated in an instant. Ecstasy to agony in the blink of an eye.
The crowd fell silent as Cakir blew the whistle for full time, save for the small pocket of delirious and disbelieving Tottenham fans in the far corner.
"I must be one of the luckiest guys on the planet I think," Eriksen told BT Sport after the match, a nervous smile plastered across his face.
"I think it was one of toughest games, it was unbelievable how it finished," Spurs boss Mauricio Pochettino told BT Sport. "I'm so happy, so proud of my players. They're fantastic, all heroes.
"I'm so happy for them, for our fans, for everyone. But we have to get ready for Saturday," he laughed, referring to Tottenham's Premier League clash against City.
In the build up, with the tie finely poised after Tottenham's 1-0 win in the first leg, most believed the Londoners held a slender advantage over their northern rivals.
But City are formidable opponents on home soil, having already scored 48 goals in all competitions so far this season. It now seems ludicrous that another four against Spurs Wednesday still wouldn't be enough.
It took just four minutes for Sterling to level the tie on aggregate, cutting inside Trippier's non-existent challenge and curling a delightful finish into the far corner.
The pre-match nerves which had lingered around the Etihad were at once banished as the stadium erupted. It was a thrilling start to the match, but nobody could have predicted what would transpire in the following six minutes.
Almost immediately from the restart, Tottenham found themselves on the edge of City's area -- the first time in the opening exchanges Spurs had even left their own half.
Dele Alli's misplaced pass was clumsily diverted into Son Heung-min's path by Aymeric Laporte, allowing the South Korean to squeeze the ball beyond Ederson.
Tottenham again had the advantage and scored the away goal City had feared, leaving them needing to score two more to advance.
But it would go from bad to worse for the hosts, as just three minutes later Son scored a stunning second, arcing his shot around Ederson and into the top corner.
City were shell-shocked and Spurs in dreamland. Guardiola's side now needed three goals to advance, surely a step too far even for free-scoring Manchester City.
But for all Tottenham's strengths across the pitch, a soft underbelly remains at crucial points in the biggest of games.
Having appeared to take complete control of this quarterfinal, nerves were again jangling just 60 seconds later as Bernardo Silva's deflected effort drew City level on the night.
And breathe. Four goals and just 11 minutes gone, one Manchester City fans reaction to Silva's goal said it all. Eyes wide and laughing incredulously, he -- like everybody else at the Etihad -- could scarcely believe what he was seeing.
So frenetic was the opening period, the lull until the game's fifth goal -- City's third -- seemed like an age. In reality, it was just nine minutes.
Silva's reverse pass found Kevin De Bruyne down the right and the Belgian's inch-perfect low cross found Sterling at the far post for an easy tap in.
Almost unbelievably, it would take until 15 minutes into the second half for the next goal to be scored.
Aguero thundered the ball past Hugo Lloris at the near post after excellent work by De Bruyne, who seemed determined to drag City through to the semifinals all by himself.
City now seemed destined for the win but Spurs, to their credit, played their way back into the match and were rewarded with two quickfire corners for their efforts.
The second proved decisive, as Llorente clumsily smothered the ball over the line.
For what seemed like an age, Turkish referee Cakir deliberated a potential handball from the Spanish striker, staring intently at the pitch-side monitors before eventually deciding to award the goal.
With stoppage time running out, and City seemingly running out of ideas, Eriksen gifted possession to Aguero in the dying seconds. The Argentine squared the ball for Sterling to score what appeared to be the winner.
Pochettino, often the coolest man in the stadium, was heartbroken and pulled his shirt over his head in attempt to block out what he had just seen.
But just when Pochettino and Spurs had that familiar sinking feeling, VAR threw them a lifeline and overturned the goal in the most spectacular of circumstances.
"It was a moment when many things happen in your head," an exhausted Pochettino said. "The decision changed everything. That is why we love football."
Liverpool ease through to semifinals
Liverpool waltzed through to a second consecutive Champions League semifinal with a 6-1 aggregate victory over Porto.
Jurgen Klopp's side traveled to Portugal leading 2-0 after the first leg and asserted their dominance inside half an hour thanks to Sadio Mane's instinctive goal.
Mohamed Salah put the tie beyond doubt in the second half, before Eder Militao scored a consolation for the hosts.
In the closing moments, Roberto Firmino and Virgil van Dijk sealed an emphatic victory.