Brazilian Maicon scored twice to steer Inter to a 2-1 win at Siena on Saturday to claim their eighth consecutive league victory and ensure they have a comfortable lead ahead of Serie A's three-week winter break.
The defender dinked in the winner seven minutes from time after Houssine Kharja's header just before halftime had cancelled out his 35th-minute opener.
Champions Inter have 42 points from 17 games, nine points more than second-placed Juventus who visit Atalanta on Sunday.
"We didn't deserve to win this match," Inter coach Jose Mourinho told Sky television. "We had fantastic spirit (while) not playing well, not playing better than the opposition.
"You have to win some games like this to win the championship."
Earlier, Lazio beat Palermo 1-0 with a 66th-minute goal by substitute Tommaso Rocchi to record their first league win in six weeks and rise to sixth.
Palermo, who almost equalised when Mark Bresciano blasted a good chance wide, had defender Federico Balzaretti sent off for a second yellow card in the 72nd minute.
Siena showed no sign of being overawed by Inter or of being distracted by this week's news that president Giovanni Lombardi Stronati was under investigation for real-estate fraud.
Moroccan midfielder Kharja had a shot well saved and forward Mario Frick missed two early opportunities.
The hosts seemed to be containing Inter with some robust tackling until Maicon nipped in behind their back line to tap the ball in from a corner.
PLUGGING AWAY
Siena kept plugging away though and Kharja did well to beat defender Maxwell to a Cristiano Del Grosso cross and equalise.
Sweden striker Zlatan Ibrahimovic hit the crossbar with a powerful strike from outside the area just before the hour but Mourinho's side still struggled to establish supremacy in the second half.
Siena's Massimo Maccarone had a shot cleared from close range shortly before Maicon ran on to a back-heel from Maxwell to score, although TV replays suggested he was offside.
Maxwell then produced an excellent block to deny Abdel Kader Ghezzal at the end.
AC Milan, who are three points behind Juve, entertain Udinese on Sunday with loan signing David Beckham due to attend.
Napoli, who share third place with Milan, visit struggling Torino.