A coach-less but stubborn Sampdoria put the brakes on Napoli's Serie A title ambitions in a 0-0 draw at the San Paolo on Sunday.
Napoli had hoped to close the gap on Juventus to just two points after the championship leaders were stunned 1-0 by Francesco Totti's rocket strike for Roma on Saturday.
But three days after a shock 3-0 defeat at home to Viktoria Plzen in the Europa League, Walter Mazzarri's side failed to conjure a goal for the second match in succession.
"This is not good enough," said Mazzarri. "If we want to be up there challenging we need more. We need consistency, it makes me angry when we show up and have these unexplainable performances."
Despite Juve's slip-up the champions are still four points ahead of Napoli on 51 points, with Lazio a distant third (44) ahead of their trip to Siena on Monday.
In the wake of a 3-0 Champions League win over Celtic on Wednesday Juve coach Antonio Conte had complained about fixture congestion affecting his players.
After Saturday's defeat, he said: "I hope in the future more consideration is given to teams who are representing Italy in Europe.
"But tonight we didn't play as a team and when that happens, so can defeat."
AC Milan, 2-1 winners over Parma on Friday, remained fourth and one point ahead of city rivals Inter after their crushing 4-1 defeat away at Fiorentina.
Both Adem Ljajic and Stevan Jovetic scored a brace to claim the Viola's first league win over Inter in 12 games, with Antonio Cassano scoring a late consolation for the visitors.
Fiorentina remained sixth, but are now only two points off third-placed Lazio and a potential spot in the Champions League.
"We're really happy. This kind of win really helps boost morale," Jovetic told Sky Italia.
"We have always believed" in being able to finish third "and tonight we proved it's achievable. We just need consistency."
Inter had beaten CFR Cluj 2-0 in the first leg of their Europa League last 32 tie on Thursday, but the defensive lapses that were to haunt them throughout this match became apparent early.
Inter coach Andrea Stramaccioni suggested fatigue, accumulated also in the Cup competition was the culprit.
"I saw my team in real difficulty tonight, and very tired. Tonight was just too ugly to be true," Stramaccioni told Sky Italia.
"We've paid for our efforts in the Cup more than can be imagined. Maybe that choice was my mistake."
Jovetic tested Slovenian Samir Handanovic with a shot after just three minutes and the hosts took a deserved lead in the 13th minute after Serbian striker Ljajic headed home from Jovetic's head-on in the box.
Serb midfielder Zdravko Kuzmanovic created Inter's best chance of the half after he pounced on a loose ball to send a snap shot past Emiliano Viviano's upright on 28 mimutes.
But five minutes later Montenegrin striker Jovetic capitalised on more shaky defending by strolling unchallenged along the area and sending a rocket past the rooted Handanovic into the roof of the net.
La Viola should have doubled their lead again befor half time when both Borja Valero and Alberto Aquilani forced saves from Handanovic.
Inter were found wanting again barely 10 minutes after the restart when a saving tackle from deep in defence left the ball ricocheting around the area before it fell to Jovetic, who gave Handanovic no chance from 10 metres out.
Ljajic made it 4-0 for the hosts on 65 minutes when he danced into the area after collecing Valero's pass from the left to beat Handanovic down low with a dipping shot.
