Diego Milito missed a penalty as Inter were held to a 0-0 draw by Atalanta, while Lazio fell to a 1-0 defeat to Catania in Serie A.
Milito's weak spot-kick saved by Andrea Consigli after just 21 minutes of play and Inter never looked like scoring after that as their two-game winning streak in all competitions came to an abrupt end.
The draw means the club have won just one of their past nine Serie A fixtures.
They were presented with a gilt-edged opportunity to add to that tally when referee Gabriele Gava pointed to the spot after Thomas Manfredini had tangled with Giampaolo Pazzini in the area.
Pazzini had converted a penalty after coming on as a replacement in the midweek Champions League clash with Marseille but it was strike partner Milito who was entrusted with the responsibility from 12 yards out.
The Argentine failed dismally in that regard, with his horribly tame side-footed effort easily pushed around the post by the Atalanta goalkeeper.
Meanwhile, Edy Reja's side are in serious risk of losing their grip on third place after going down to a late Catania goal at the Angelo Massimino.
The winner came with just 10 minutes to spare, as Nicola Legrottaglie rose highest to meet Francesco Lodi's corner and put the ball beyond the reach of Federico Marchetti.
More bad news ensued for the Biancocelesti, as Reja was sent to the stands for protesting at a decision made by referee Andrea Romeo.
Lazio stay in third, but are now only two points ahead of Napoli, who play Udinese in Sunday night's Calcio offering.
In other matches, Alessandro Diamanti's 81st-minute equaliser handed Bologna a point in an exciting 2-2 draw against Chievo.
Marco Andreolli gave Chievo the lead after 26 minutes before Marco Di Vaio responded on the hour-mark for Bologna.
But parity lasted just 10 minutes as Cyril Thereau put Chievo in front again, only for Diamanti's late goal to ensure both sides would share the spoils.
Chilean Mauricio Pinilla scored a hat-trick - including two from the penalty spot - as Cagliari toppled a nine-man Cesena 3-0, while Lecce and Palermo both finished with 10 men in a 1-1 stalemate.
Second-bottom club Novara won 2-0 at Siena.