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Inter was the least bad of the two teams and Lautaro Martinez's persistence paid off with the goal that brought a 1-0 victory, trimming Napoli's lead to a still-commanding 13 points with 17 rounds of games to play,
Napoli is aiming for a first Scudetto in 33 years and its 3-0 dismissal of Spezia much earlier in the day was a fresh taste of how far it is ahead of everyone else in Italy.
Milan, the reigning champion, is sixth and in free-fall. Inter sits second but if is the best of the rest Napoli can rest easy.
Nominally the away team at San Siro, the Rossoneri should have been up for this battle for city bragging rights.
Instead, tit was awful and has lost four games in a row, with this result after a 3-0 Supercoppa drubbing by Inter, a 4-0 thrashing at Lazio and a 5-2 humiliation at home at the hands of lowly Sassuolo.
Head coach Stefano Pioli cannot rest on the goodwill of last season forever and, if his team had not already piled his plate high with more immediate reasons for concern, he would surely be panicking about his job.
Seven games have passed since Milan last won and the four-game streak of defeats is its longest in six years, since Vincenzo Montella was at the helm.
Inter stripped its captain Milan Skriniar of the armband before this game, with Martinez taking on the responsibility.
It is anticipated Skriniar will leave as a free agent to join Paris Saint-Germain in July but he played here and celebrated along with the Curva Nord at the final whistle as Simone Inzaghi's team took the spoils.
Inter owed the win to Martinez, the man who can't stop scoring against Milan.
The Argentine striker had gone repeatedly close to a breakthrough in the early stages, rifling over in the second minute, drawing a smart save from Ciprian Tatarusanu four minutes later, then heading six inches wide from Skriniar's clever ball into the box in the 10th minute.
Skriniar headed over from former Milan midfielder Hakan Calhanoglu's free-kick as a chugging game chugged on.
Inter then provided a rare moment of attacking verve, breaking at speed before Martinez was chopped down by Pierre Kalulu for the most obvious yellow card of the season.
The breakthrough was clearly coming and arrived in the 34th minute when Martinez, dancing between statues at the near post, flicked in a corner whipped in from the left by Calhanoglu.
At the break Inter led the shot count nine to zero. The difference of nine was the widest in the first half of a game between these teams since at least 2004-05.
Perhaps the writing had been on the wall for Milan when Pioli spoke with such enthusiasm before the game of the impending return of Zlatan Ibrahimovic, a 41-year-old who last scored a Serie A goal in January 2022 and has since undergone major knee surgery.
Martinez now has seven goals against Milan, the club's second-highest scorer in the derby, beaten only by the haul of 11 by Stefano Nyers, a Nerazzurri star of the 1940s and 1950s.
But what would the second half bring? More slop, mainly.
Star attacker Rafael Leao, only a substitute for the second game in a row, came off the Milan bench and was booked for slapping Nicolo Barella.
Nothing Milan attempted came off and it was a wonder it remained in touch. Its first goal attempt of the game arrived in the 58th minute, Olivier Giroud heading six yards over the crossbar.
Giroud then had a glorious chance to equalise in the 76th minute, after a flash of Leao at his dribbling best down the left, but the French striker took a terrible first touch and the chance disappeared.
Leao walked away, utterly despondent judging by his body language.
Former Chelsea and Arsenal man Giroud had been looking to become the first Milan player in the three-points-per-win era, which began in 1994-95, to score in his first three league derbies against Inter. Presented with the ball on his favoured left foot and only 12 yards from goal, he didn't even get a shot away.
Inter substitute Romelu Lukaku was then denied a goal when he was ruled to have fouled Malick Thiaw before firing past Tatarusanu.
So it remained 1-0, Inter closing out a 68th league victory over its city foe.
That is now a record number of defeats against a single opponent for Milan, with 67 losses to Inter and Juventus having previously been a joint high.
And it's Napoli's title, just as Roma boss Jose Mourinho said last week. They're the Special Ones this season.