Serie A leaders Inter Milan came from behind to beat city rivals AC Milan 2-1 on Sunday and extend their advantage over AS Roma to seven points.
Argentine duo Julio Cruz and Esteban Cambiasso scored in either half to cancel out Andrea Pirlo's 18th minute opener.
Unbeaten Inter's sixth consecutive league win gives them 43 points from 17 games, while Roma are on 36 after beating Sampdoria 2-0 on Saturday thanks to captain Francesco Totti's brace.
Defeat means Milan have still not won against Italian opposition at the San Siro this season and are 25 points behind Inter in 12th place, but with three games in hand.
Third-placed Juventus have 35 points after beating Siena 2-0 in Turin on Sunday.
"I think we played a good match and deserved to win," Inter coach Roberto Mancini told Sky television.
"They didn't have too many chances, except at the end. It's important that we have finished [the year] with a seven-point lead despite all the [injury] problems we have had."
At the San Siro, Inter's Sweden striker Zlatan Ibrahimovic volleyed the first chance over after being set up by Maicon in the fourth minute.
But the match really came to life when Pirlo found the top corner with a free kick from the edge of the box.
Inter surged back and their Chilean midfielder Luis Jimenez almost levelled by was denied by the crossbar.
The champions equalised in the 36th when Cruz controlled a Cambiasso pass and drove the ball home despite the attention of three defenders.
Cambiasso decided the match in the 63rd minute, scoring with a shot from outside the box that deceived Brazilian keeper Dida.
Milan almost grabbed a point with a diagonal shot from Kaka that produced a fine save from Julio Cesar, while Massimo Ambrosini fluffed a great chance with the goal gaping in the closing stages.
"It was a very well balanced derby that was decided by individual incidents," Milan coach Carlo Ancelotti said.
"We should have had a penalty when we were 1-0 up for a foul by Cambiasso on Kaka.
"A foul on Pirlo was not spotted at the start of the move that led to Cambiasso's goal and Dida made a mistake with a controllable shot. A draw would have been fairer."
In Turin, Bosnian midfielder Hasan Salihamidzic tapped Juve in front just after the half-hour mark after a Pavel Nedved cross caused panic in the Siena defence.
France forward David Trezeguet doubled the lead and claimed his 13th goal of season with a right-footed shot from the edge of the area in the second half.
Fourth-placed Udinese were held to a 2-2 home draw by Empoli.
Mario Santana and Adrian Mutu scored braces as fifth-placed Fiorentina thrashed basement club Cagliari 5-1.
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