Inter Milan stretched their Serie A lead to eight points after the champions won 3-0 at Lecce and AC Milan could only draw 1-1 at home to bottom side Reggina on Saturday.
The Milan derby takes place on Feb. 15 and Inter will be licking their lips at the thought of going 11 points clear of their city rivals, who won the first encounter this term 1-0.
Second-placed Milan will struggle to repeat that performance if they play like they did against Reggina, who deservedly went ahead on 33 minutes when Davide Di Gennaro fired in after Bernardo Corradi's assist.
Kaka equalised with a 67th-minute penalty but soon went off with an injured foot and is a doubt for the derby after being taken to hospital for tests.
The Brazilian also hit the post and Clarence Seedorf had a goal ruled out when he used his hand to net the rebound.
Milan, fifth last season because of their shaky home form, had the majority of the play but a raft of David Beckham crosses went begging with the team lacking height in the box.
Ronaldinho started after almost a month on the bench but was substituted in the second half having managed just one good run.
"It was a difficult and nervous match for all," Milan coach Carlo Ancelotti told Sky television.
"Maybe we thought the goal would come sooner or later. Reggina had very tall defenders and we had little presence in the box."
Inter were ahead on 12 minutes when Swedish talisman Zlatan Ibrahimovic slammed his 14th league goal of the campaign after the busy Esteban Cambiasso laid the ball off having pretended to shoot.
Maicon missed from close range after the break but Luis Figo, 36, made sure they would not regret it when he headed in a cross from teenage left back Davide Santon in the 71st minute.
Former Portugal captain Figo, used little this term having put off retirement at the end of last season, had not scored a league goal since a 3-0 win over Sampdoria in Sept. 2007.
Dejan Stankovic nodded in Maicon's free kick late on having again been a driving force for Inter, who operated with the Serbian and Figo behind lone front man Ibrahimovic.
The champions nervously drew 1-1 at home to Torino last weekend and coach Jose Mourinho was impressed with the response.
"We played a great match," he said. "We saw a compact team with great spirit and fantastic concentration.
"I'm satisfied with the points but also with how the team reacted after last weekend's result."
Fourth-from-bottom Lecce, four points above the drop zone, had some decent chances to equalise at 1-0 with defender Guglielmo Stendardo seeing his header saved by Julio Cesar.
Third-placed Juventus, 10 points adrift of Inter and two behind Milan, visit Catania on Sunday.