Cagliari end Viola's purple patch
Vincenzo Montella's side had taken 14 points from a possible 18 in their last six games following a 2-1 reverse to Roma at the Olympic Stadium in early December.
But La Viola's run, which had put them within three points of third-placed Napoli in the first Champions League spot, was ended when Mauricio Pinilla slotted home from the penalty spot for the Sardinians just before half-time.
Montella complained Cagliari had effectively shut up shop in the second half to protect their lead.
"I don't usually complain... but we didn't lose because of refereeing decisions -- Cagliari simply played to defend after they had taken the lead," he said.
In Saturday's other early game struggling Udinese ended a run of four straight defeats with a 2-0 away win at relegation-threatened Bologna to claim their first league win of the year.
Udinese, who had finished an impressive fifth place last season, belied their poor league form in this campaign to oust AC Milan from the quarter-finals of the Italian Cup last week and set up a two-leg semi-final clash with Fiorentina.
Veteran striker Antonio Di Natale's penalty put the northerners within sight of their first victory since a 2-1 away win at Livorno prior to Christmas.
Nico Lopez then settled last-minute nerves with an injury-time strike.
Udinese's win was all the more sweet as teenaged goalkeeper Simone Scuffet kept a clean sheet after being called between the posts when Zeljko Brkic suffered a problem in the warm-up.
"It was an incredible feeling," Scuffet told Sky Sport Italia.
"The win today will help push us clear of the danger zone so we're a little more relaxed."
Fiorentina remain in fourth although Napoli could extend their lead over the Florence side to six points with victory away to Atalanta on Sunday.
Cagliari and Udinese moved up one place to 13th and 14th respectively ahead of the remainder of the weekend fixtures while Bologna could drop into the relegation zone depending on Sassuolo and Livorno's respective results.
Milan will be looking for their third consecutive league win under coach Clarence Seedorf at home to Torino in Saturday's late game.
Juventus, who sit top with a six-point lead on Roma, host Inter Milan in Sunday's eagerly-awaited late game with Roma hosting Parma at the Olympic Stadium earlier in the day.