With 75 minutes gone and Manchester United making little headway into Manchester City's one-goal advantage, the visitors finally came to life.
Or at least, their septuagenarian manager did. After Nigel De Jong challenged Danny Welbeck with enough ferocity to prompt a booking, Sir Alex Ferguson advanced into the City technical area like a nightclub bouncer ready to attend to some business.
With fists clenched, invective streaming out of his mouth and bristling with intent, his target was Roberto Mancini.
A shouting match ensued, Ferguson motioned with his right hand that Mancini should quieten down and the Italian, his normally immaculate locks buffeted by the a strong wind, refused to take a backward step and responded in identical fashion to the Govan Godfather.
Just as it was about to get tasty, assorted backroom staff members led by Mancini's assistant David Platt dragged the warring pair apart.
IN THE DRIVING SEAT Man City's remaining fixtures May 6 Newcastle (A) May 13 QPR (H)
ON THE BACK FOOT Man Utd's remaining fixtures May 6 Swansea (H) May 13 Sunderland (A)
Afterwards, Ferguson blamed Mancini for provoking the flare-up: "He was badgering the officials all game – the fourth official and the referee. He can't be complaining about referees tonight".
It was not lost on most observers that the feisty Scot showed more fight in that moment than his passive team had done over the 90 minutes.
Ferguson has enough credit in the bank to oversee a decade of consecutive derby defeats, never mind two in a season.
But this was not a night for his volumes of scrapbooks. He picked the wrong team, organised them into a cautious formation and saw his side fail to produce a shot on target for the first time in a Premier League match in three years.
A victory by a wider margin would not have flattered City, who made up for what they lacked in final-third dazzle with energy, power and hunger, epitomised by the outstanding Yaya Toure and Vincent Kompany.
Ferguson's tactics failed miserably. He chose his European away-day formation, setting his team out in 4-5-1 with Wayne Rooney up front alone, and Ryan Giggs and Park-Ji Sung given starts ahead of in-form pair Danny Welbeck and Antonio Valencia.
It looked curious beforehand. It seemed even more perverse afterwards. Park played in the centre, tracking Toure, while Giggs was fielded in a narrow left-sided position. Neither could influence proceedings and the ineffective South Korean was withdrawn after a pitiful 57 minutes. It could easily have been at half-time.
Given that United had eight days to prepare for this showdown and have had no European commitments for two months, no blame could be attached to an unyielding fixture list.
Ferguson, whose reputation as a connoisseur of cavalier football is well merited, picked a team to not lose. It almost felt like a betrayal.
The tension of the title run-in has been getting to United. Twice, the United boss, concerned that the pressure was fraying the nerves of his younger players in particular, has taken his team on mini-breaks to get away from the suffocating cauldron of Manchester – first to the golfing mecca of St Andrew's and then to Cardiff at the weekend.
This magical mystery tour of the Celtic lands does not appear to have had the restorative effect he would have hoped.
After opening up an eight-point lead at the top of the Premier League, United have taken only four points from their last four league games, giving City hope by losing to Wigan Athletic, failing to hold on to a two-goal lead against Everton and now this derby surrender. Any other title contenders would be accused of bottling it.
This felt like a night when the awesome powers of the finest British manager of his generation – and perhaps ever – were on the wane.
For the biggest match of the season, why omit Valencia, who has been as influential as any United player in the second half of the campaign? Likewise, the demotion of Welbeck was puzzling. Furthermore, why was arch-predator Javier Hernandez not thrown on for the last 10 minutes to sniff an equaliser?
The seething desire and hunger is undimmed, but it will pain Ferguson to the core that his team were so listless and could not even produce the kind of late fightback for which they have been famed for two decades.
PREMIER LEAGUE TABLE WITH TWO GAMES TO GO
P W D L F A GD Pts Manchester City 36 26 5 5 88 27 +61 83 Manchester Utd 36 26 5 5 86 33 +53 83
By contrast, Mancini has had the sure touch of a clinical strategist over the last few weeks.
His game plans have worked. He has refused to get drawn into mind games, other than bowing to United's history and comments about Sunderland and Swansea City being "easy, easy" games for Ferguson's team will ensure the players of Brendan Rodgers and Martin O'Neill are fired up by their managers. Even Carlos Tevez and Mario Balotelli have stopped giving him headaches.
The suave Italian picked an unchanged XI and his ambition was rewarded with a performance shimmering with intent as City's players were driven on by impassioned home support. All the adventure came from the sky blue shirts.
The key men were Samir Nasri and Pablo Zabaleta on the right flank. With Giggs so narrow and lacking the mobility to shuttle up and down, the two players were able to manoeuvre two against one situations against Patrice Evra, whose growing deficiencies were again exposed.
There were murmurs of disapproval when Mancini responded to the introduction of Welbeck by withdrawing Tevez for Nigel de Jong but it worked beautifully, as it enabled City to keep a lid on Rooney and freed up Toure to move 20 yards up the field and scare United's defence with his powerful runs.
Like so much on a night when the momentum behind City's title challenge felt almost unstoppable, Mancini got it spot on. Ferguson did not.