A spectacular first-half header from Argentine striker Lautaro Martinez propelled Inter Milan to a 1-0 victory over local rivals AC Milan at a raucous San Siro on Sunday, condemning Stefano Pioli's side to a third consecutive defeat in Serie A.
Inter, who also beat Milan in the final of the Italian Supercup last month, remains second in the standings with 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.
"It was certainly a game for which I congratulated my team," Inter coach Simone Inzaghi told DAZN. "They were concentrated, determined, careful; we have dominated.
"Winning two derbies in 20 days makes me very satisfied. We look forward with confidence."
The unmarked Lautaro produced Inter's first serious threat in the sixth minute, but Milan keeper 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 goal attempt, 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.
"The negative games have robbed us of confidence," Pioli said. "We are not so mature with some absentees and other players who are not at their best.
"We have to start over as of tonight. It's a defeat that hurts, but we have rediscovered the solidity that can allow us to return to our level."
In the next round of games, Milan hosts Torino on Friday, while Inter travels 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, travels to Verona on Monday, while Juventus visit Salernitana on Tuesday.
