Inter Milan have a seven-point lead and a game in hand at the top of Serie A after their closest challengers Juventus fell to a shock 1-0 home defeat to Udinese on Monday.
Juve's title hopes took a hit at the Allianz Stadium in Turin after Lautaro Giannetti poked home the only goal of the game in the 25th minute.
Argentine defender Giannetti's strike was his first since signing for Udinese last month and gave his team just their third league win of the season.
Gabriele Cioffi's team are three points above the relegation zone after consigning disappointing Juve to their second straight defeat after last weekend's narrow loss at Inter.
Juve, missing injured striker Dusan Vlahovic, can have no complaints as they created very little and were booed off by their home fans frustrated at the poor performance and their team's wilting title challenge.
The closest Juve came to equalising when Arkadiusz Milik nodded in from a corner but the goal was ruled out for Federico Chiesa's cross going out of play.
A first home defeat of the season for Massimiliano's side could hardly have come at a better time for Inter, who beat Roma 4-2 on Saturday.
The league leaders have a game in hand and have a tough run of fixtures, including Atletico Madrid in the last 16 of the Champions League, coming up over the next few weeks.
Juve meanwhile are now looking over their shoulders as AC Milan are just a point back in third thanks to their 1-0 win over Napoli on Sunday night.
