With Juventus drawing 0-0 with Lazio on Saturday night, second-placed Inter could have moved to within two points of the pacesetters with a victory on Sunday night.
However, they not only failed to win, but they needed an 82nd-minute Davide Astori own goal just to salvage a point after a Marco Sau brace had seen impressive Cagliari bounce back from an early deficit to take the lead.
Inter had seen their six-match Serie A winning streak ended by last week's 3-2 loss at Atalanta and their hopes of getting back on track today looked good when Rodrigo Palacio headed them in front after just 10 minutes.
But that was before Sau's double and Astori's late intervention.
Also on Saturday, Stephan El Shaarawy rode to AC Milan's rescue once again as his clinical strikes spared the Rossoneri's blushes in a 2-2 draw at Napoli.
Meanwhile, Alberto Aquilani scored twice and set up another as in-form Fiorentina brushed aside 10-man Atalanta 4-1 to record their fifth successive Serie A victory.
Aquilani took centre-stage after Atalanta's Giacomo Bonaventura had cancelled out Gonzalo Rodriguez's early opener for Fiorentina.
The ex-Liverpool man put the Viola in control with two goals with Atalanta also having Luca Cigarini sent off in between.
Aquilani then turned provider at the start of the second half with a cross that Luca Toni headed home.
Bologna picked up three vital points by beating Palermo 3-0 in a relegation scrap which included four red cards.
Alberto Gilardino put Stefano Pioli's side in front with Manolo Gabbiadini making it two from the penalty spot.
Another penalty scored by Alessandro Diamanti early in the second half completed the scoring, but the referee's work was only just beginning as he first sent of Samir Ujkani for giving away the spot-kick.
He was followed by Edgar Barreto, Saphir Taider and Carlos Labrin, in chronological order, with Bologna ending the game with 10 men and three points and Palermo finishing with eight and zero.
Raffaele Palladino ensured Parma would return home with a precious point from a 2-2 draw at Udinese.
Antonio Di Natale made it eight Serie A goals for the season before Marco Marchionni scored.
Roberto Pereyra nudged Udinese ahead once again but Palladino ensured it finished 2-2.
Sergio Almiron's second-half double earned Catania a 2-1 win over Chievo at the Stadio Angelo Massimino.
Almiron opened the scoring six minutes into the second half and curled home a crucial second goal five minutes from time before Marco Andreolli's stoppage-time consolation.
Francesco Valiani's first-half goal saw Siena win 1-0 over nine-man Pescara.
Serse Cosmi was the first dismissal on a fractious afternoon in Tuscany, the Bianconeri head coach seeing red after Ante Vukusic missed a penalty just moments before the break.
But Valiani's regulation finish on the half hour settled a game which saw Pescara team-mates Marco Capuano and Damiano Zanon dismissed.
The bragging rights are back with Sampdoria for the first time since 2010 as Mauro Icardi's late strike sealed a 3-1 victory over Genoa in the Derby della Lanterna.
There was more at stake than usual in the showdown at the Stadio Luigi Ferraris, with Samp on a run of seven straight defeats and Genoa having lost five in a row.
Andrea Poli certainly lightened the load on Doria boss Ciro Ferrara with a fine opener before Genoa's Cesare Bovo gifted the Blucerchiati a 2-0 lead in comical fashion with an own goal.
A much improved Genoa hit back through Ciro Immobile after the break, but Icardi ensured an uncertain future for Grifone boss Luigi Del Neri with his strike at the death.
And on Monday, Roma host Torino, with ust three points separating the two sides in the Serie A.
Saturday's Serie A Fixtures
Sunday's Serie A Fixtures
Monday's Serie A Fixtures