AC Milan humiliated their city rivals Inter Milan by winning the Derby della Madonnina 3-0 at San Siro.
The Rossoneri had not won the showpiece Serie A match in three attempts and entered Sunday's meeting in a distant sixth place, while Inter were eight points better off and still in the title race.
The script was flipped, however, with Alex powering home the opener before Inter captain Mauro Icardi -- left on the bench by Roberto Mancini -- missed a second-half penalty.
While the Nerazzurri were still sulking, Carlos Bacca and M'Baye Niang fired Milan to their best derby victory in five years.
Winning the Scudetto now looks a tall order for an Inter side who have not won in four outings. Milan, meanwhile, can begin to dream about playing Champions League football once again.
For many the spectacle of the derby is not confined to the pitch, but represented in the stands, where fans of both sides unveil banners supporting their own club and condemning the other.
While Nerazzurri ultras described their team's play as "more beautiful than a dream," hardcore Milan fans recalled the days of Mark Hateley, who was depicted towering over Inter's Fulvio Collovati.
A lot has changed since 1984 but the derby means just as much to the city today, and the latest edition started with real fervour.
Inter provided much of the early excitement, with new signing Eder flattering to deceive as he fluffed a chance to put an easy header away at the far post.
Marcelo Brozovic soon let rip from distance, but Milan's teenage goalkeeper Gianluigi Donnarumma, who turns 17 at the end of February, smothering the sighter with relative ease.
Samir Handanovic repeated the trick as Juraj Kucka came knocking at the other end and Donnarumma was left untroubled when Stevan Jovetic tried his luck with a left-footed effort.
Less than 60 seconds later Alex gave Milan the lead. In emulating Hateley's ball-winning ascent, the Brazilian leapt above Davide Santon to become the latest South American to score in the derby.
Milan enjoyed the moment but were nearly left embarrassed by Donnarumma's poor decision.
The 16-year-old raced off his line to deal with Ivan Perisic, allowing Juan Jesus to swing the ball into an empty penalty box. Ignazio Abate somehow made up the yards to snatch the ball off Eder's feet.
Donnarumma more than covered for his mistake when the second half began, though. This time he had no option but to rush out and engage Eder -- his touch stopped the ball dead, but felled the striker.
Referee Antonio Damato promptly signalled for a penalty before changing his mind, infuriating Inter boss Mancini who was sent to the stands for his protests.
The exiled Icardi shot daggers at his coach, who soon sent down an order for the Argentinian to warm up and replace the ineffective Jovetic.
It proved a timely substitution as Icardi was needed for a spot-kick after 70 minutes, with Alex bringing him down as he tried to reach Adem Ljajic's through-ball.
The smirk disappeared from his face, however, as his penalty beat Donnarumma but not the post.
Milan took heart from their rivals' despondency and, scenting blood, went in for the kill.
Bacca made it 2-0 when meeting Niang's cross with a classy volley and barely four minutes later the Rossoneri extended their lead even further.
Provider turned goalscorer as Giacomo Bonaventura exploited an error from Santon to tee-up Niang, who saw his first attempt saved by Handanovic.
The Slovenian could do nothing about the second.