Surely bound to end a 33-year league title wait at Milan's expense, Luciano Spalletti's Napoli kicked off this encounter 23 points clear of their visitors, who themselves went into the game in poor form, with only Lecce having suffered more defeats across the previous nine league rounds.
As such, it was a profound shock when the Rossoneri went ahead within 17 minutes. Brahim Diaz picked up the ball on the half-turn in a pocket of space, before threading a perfectly-weighted pass into the path of Leao, who dinked the ball over the keeper for his first goal since mid-January.
Just eight minutes later, it was Diaz again at the heart of everything good about AC Milan in the first half.
Ismael Bennacer hung up a cross for Olivier Giroud, but the forward could not make a clean connection. The ball subsequently dropped to Diaz inside the box, and the Spaniard sensationally sat down two defenders before finishing high into the roof of the net.
As expected from the champions-elect, Napoli came out in the second half with added verve, but little managed to stick offensively without their injured talisman, Victor Osimhen. This lack of a focal point up front was no more exploited than when Milan put themselves three goals to the good midway through the second half.
Sandro Tonali released Leao down the left, and the Portuguese poacher unleashed a devastating finish past Alex Meret.
Napoli had conceded just nine goals at home all season before Milan arrived in Naples, and just five minutes later, the rout was complete. Substitute Alexis Saelemaekers waltzed through the Napoli defence before finishing through Meret’s legs.
Nobody present at the Stadio Diego Armando Maradona could believe what they had just seen, and the result gives a pointed reminder from the current champions that they are still a force to be reckoned with in Italian football.
Thoroughly beaten on the night, Napoli must now wait ten agonising days for any chance of revenge, as they travel to Milan for the first leg of the Champions League quarter-finals.
Flashscore Man of the Match: Rafael Leao (AC Milan)
