Lautaro Martinez scored the only goal as Inter Milan beat city rivals AC Milan at the San Siro.
Second-placed Inter remain 13 points behind Serie A leaders Napoli, who won 3-0 at Spezia earlier in the day.
Captain Martinez headed the only goal of the game from a corner by former Milan midfielder Hakan Calhanoglu.
Inter had two goals disallowed late on - substitute Romelu Lukaku's effort was ruled out for a foul on Malick Thiaw and Martinez's strike for offside.
A long video assistant referee decision upheld the decision to rule out Argentina striker Martinez's effort because he was offside when he ran onto Lukaku's flick to slot home.
Milan, who dropped to sixth in Serie A this weekend, have lost five of their last six games in all competitions.