Lautaro Martinez's purple patch continued as Inter Milan consigned sorry AC Milan to a fourth successive defeat with a 1-0 derby victory.
The in-form Martinez, handed the captain's armband by Simone Inzaghi, took his goal tally for the season to 15 by opening the scoring with a header in a totally one-sided first half at San Siro on Sunday.
Milan failed to register a shot on target in a lacklustre display that increased their winless run to seven matches, putting more pressure on head coach Stefano Pioli.
Second-placed Inter moved five points clear of their city rivals behind runaway Serie A leaders Napoli, with the Rossoneri down in sixth spot as their dismal start to the year continued.
Martinez forced a magnificent reflex save from Ciprian Tatarusanu and also headed just wide in a blistering start from the Nerazzurri.
Inzaghi's side were dominant, but referee Davide Massa correctly refused to award them a penalty when Edin Dzeko went down too easily under a challenge from Matteo Gabbia.
It seemed to be only a matter of time before Inter were in front, though, and the opening goal duly arrived in the 34th minute, when the lively Martinez rose above Simon Kjaer to nod in Hakan Calhanoglu's corner from the left.
It remained one-way traffic following the break, Tatarusanu palming away Martinez's shot at his near post after Pioli introduced Rafael Leao and Alexis Saelemaekers.
Olivier Giroud finally had Milan's first attempt after 57 minutes but his header sailed over the crossbar and was never going to trouble Andre Onana.
Further goals instead appeared more likely at the other end.
Romelu Lukaku had one ruled out when he was deemed to have dragged down Malick Thiaw and Martinez had another chalked off for a marginal offside, but his first-half finish proved decisive.
What does it mean? Pioli under scrutiny after Milan's slump continues
Milan have lost four games in a row for the first time since Vincenzo Montella was in charge six years ago as they failed to exact revenge for a 3-0 Supercoppa Italiana defeat to Inter last month.
Following a woeful January in which they conceded 18 goals, the most they have shipped in a single month of a year in their history in all competitions (since 1929-30), the champions are relinquishing their title with a whimper.
As the under-pressure Pioli contemplates how to arrest their slide, Inter have won three games on the spin in all competitions and sit 13 points adrift of a seemingly unstoppable Napoli side.
Martinez the main man
Argentina striker Martinez has scored seven goals in nine appearances for Inter since returning to the club as a World Cup winner.
He led by example as skipper after being given that honour due to Milan Skriniar, who was rock solid at centre-half, being bound for Paris Saint-Germain at the end of the season.
Martinez has scored seven times against Milan, with only Stefano Nyers (11) finding the back of the net more times for Inter in derby clashes.
Kjaer not commanding
Although Martinez took his goal well, Denmark centre-back Kjaer made it too easy for him. He was a long way off his commanding best, winning only one duel as Milan were second-best all over the park.
What's next?
A wounded Milan are at home to Torino on Friday, while Inter are at Sampdoria a week on Monday.
