While Milan's fightback against Frosinone tightened their grip on third place in the standings, their visitors scraped past Hellas Verona thanks to a late moment of magic.
Match preview
Keeping themselves on the coat-tails of city rivals Inter Milan, who lead Serie A by four points following last week's Derby d'Italia, Milan turned things around in the final 20 minutes of their most recent outing, against mid-table Frosinone.
Down 2-1 with time running out, Matteo Gabbia was the unlikely scorer of the Rossoneri's second goal - Olivier Giroud had earlier taken his tally of goal involvements to 19 in 20 league appearances, which is already the French striker's most in a single Serie A season.
Then, Giroud's deputy Luka Jovic left the bench to produce a late winner, making Milan the team with most goals via substitutes (10) in Italy's top flight; and since the start of December, Jovic has the best scoring ratio across Europe's top five leagues - one every 41 minutes.
Having previously beaten Udinese 3-2 and drawn 2-2 with Bologna, Stefano Pioli's side have certainly started 2024 with renewed confidence in the final third, though they have also conceded at least twice in each of their last three matches.
Over the last eight rounds, they boast a 2.5 points-per-game average in Serie A - fewer only than Inter over that period - and cannot be entirely ruled out of this year's Scudetto race.
Sitting 10 points clear of fourth place heading in to the weekend, Milan are comfortably on course for a Champions League return, while they kick off their Europa League campaign against Rennes next Thursday.
First, though, the second-city club host a side they drew 2-2 with in October's reverse fixture, when Giroud was at the double against one of his favourite opponents.
In addition to their loss in last season's Champions League quarter-finals, Napoli were also beaten 4-0 by Milan in an otherwise successful 2022-23 campaign that saw them replace the Rossoneri as Italian champions.
Of course, the Partenopei's title defence has hardly gone to plan, with former boss Walter Mazzarri now back on the bench as successor to the unpopular Rudi Garcia - but to little effect.
After a drab goalless draw that saw them fail to record a single shot on target against Lazio at Stadio Olimpico, Napoli were in some danger of slipping down to 10th place ahead of last week's home game versus Verona.
Despite dominating the ball against relegation-threatened opposition, it took a tremendous Khvicha Kvaratskhelia strike to spare the Scudetto-holders' blushes in an unconvincing 2-1 win that still left them four points shy of the top four.
While they have consistently struggled to match expectations in front of their own fans, Mazzarri's men have also started to falter on the road: they are goalless in four away matches stretching back to November, equalling the club's worst such run since 1979.
Soon to test themselves against another fallen giant, as Barcelona await in the Champions League's last 16, Napoli must first try their luck at San Siro, where they are on a three-match winning streak against Milan.
AC Milan Serie A form:
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D
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AC Milan form (all competitions):
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L
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Napoli Serie A form:
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D
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Napoli form (all competitions):
L
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Team News
Although Malick Thiaw has returned to full training after being sidelined by a hamstring injury for two months, Milan's defensive department continues to be understaffed, as Pierre Kalulu and Fikayo Tomori are still unavailable.
With Thiaw well short of match-fitness, Matteo Gabbia and Simon Kjaer should remain at the heart of the Rossoneri's back four on Sunday, but Stefano Pioli must make one enforced change following four games with the same starting XI: Tijjani Reijnders is suspended, so Ismael Bennacer comes in to midfield.
While Luka Jovic is enjoying a purple patch, Olivier Giroud again leads the line, having previously scored five goals against Napoli in all competitions - he needs one more (or an assist) this week to record 20 goal involvements for the first season since 2015-16.
Milan's side has been settled, but it remains to be seen whether Napoli will again ditch their renowned 4-3-3 formation for a three-man defence and wing-backs; Walter Mazzarri has wavered between the two options of late.
The visitors could welcome Alex Meret and Mathias Olivera back to the bench following injury layoffs, while Piotr Zielinski - who is expected to leave on a free transfer this summer - returns from a muscle strain. However, Mario Rui is absent due to suspension, so January signing Pasquale Mazzocchi may feature on the left flank.
With Victor Osimhen representing Nigeria in the Africa Cup of Nations final - alongside Milan's Samu Chukwueze - Giovanni Simeone is favourite to start up front, supported by last week's matchwinner Khvicha Kvaratskhelia and Matteo Politano. The latter has scored five goals against Milan in Serie A, and four have come at San Siro.
AC Milan possible starting lineup:
Maignan; Calabria, Kjaer, Gabbia, Hernandez; Bennacer, Adli; Pulisic, Loftus-Cheek, Leao; Giroud
Napoli possible starting lineup:
Gollini; Ostigard, Rrahmani, Jesus; Di Lorenzo, Anguissa, Lobotka, Mazzocchi; Politano, Simeone, Kvaratskhelia