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 Udinese are three points above the relegation zone after consigning disappointing Juve to their second straight defeat after last weekend's narrow loss at Inter.
"Unfortunately it's not a great period for us right now," said Juve coach Massimiliano Allegri.
"We need to get our focus back because we still need to get a lot more points if we're going to qualify for the Champions League."
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 was near the hour mark when Arkadiusz Milik nodded in from a corner, but the goal was ruled out for Federico Chiesa's cross going out of play.
{{/usCountry}}The closest Juve came to equalising was near the hour mark when Arkadiusz Milik nodded in from a corner, but the goal was ruled out for Federico Chiesa's cross going out of play.
{{/usCountry}}A first home defeat of the season for Allegri's side could hardly have come at a better time for Inter, who beat Roma 4-2 on Saturday.
The league leaders have a tough run of fixtures, including Atletico Madrid in the last 16 of the Champions League and their game in hand with Atalanta, coming up over the next few weeks.
Juve meanwhile are now looking over their shoulders as AC Milan are just a point behind them in third thanks to a 1-0 win over Napoli on Sunday night.