A first-half header by striker Lautaro Martinez gave Inter Milan a 1-0 victory over locals rivals AC Milan at a sold-out San Siro on Sunday, condemning Stefano Pioli's side to their third consecutive loss in Serie A.
Inter remain second in the standings on 43 points, 13 behind runaway leaders Napoli. Milan, who have now lost four games in a row in all competitions, dropped to sixth, on 38 points.
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The unmarked Lautaro produced Inter's first serious threat in the sixth minute but Milan goalkeeper Ciprian Tatarusanu saved his header from going inside the left-hand post.
The Argentine striker gave Inter the lead in the 34th minute when he nodded in a corner from close range for his 12th goal this season.
Inter dominated the first half with 76% of possession and prevented Milan from any attempt on goal, but they lost some of their intensity after halftime.
Milan responded with their first shot just before the hour mark but a mid-range header by striker Olivier Giroud went over the crossbar.
They followed up with close chances by substitute Brahim Diaz, whose shot was stopped by Inter goalkeeper Andre Onana, and another opportunity by Giroud whose free kick went wide.
Inter then had the ball inside the net twice but striker Romelu Lukaku was denied a goal for a foul, while Lautaro's strike was ruled out for offside.
Lukaku had one more chance to double Inter's advantage in stoppage time when he fired a shot from the edge of the box but Tatarusanu made a superb save to keep it out.
In the next round of games, Milan host Torino on Friday, while Inter travel to Sampdoria on Feb. 13.
Earlier on Sunday, Torino beat Udinese 1-0 to overtake them in the standings and move to seventh on 30 points. Bologna, in ninth, beat 12th-placed Fiorentina 2-1.
Fourth-placed Lazio, on 38 points, travel to Verona on Monday while Juventus visit Salernitana on Tuesday.