The Italians\' winning run in all competitions was ended with a 3-2 loss on Sunday night. They now trail leaders Juventus by four points.
Milan: Inter Milan's 10-game winning run in all competitions was ended Sunday with a 3-2 loss at Atalanta in Serie A.
After ending Juventus' 49-game unbeaten run in Serie A last weekend, Inter have slipped four points behind the leaders and defending champions. They is also only a point ahead of Napoli, who beat Genoa 4-2.
Lazio came from behind to beat Roma 3-2 in a rain-drenched derby, and AC Milan missed a penalty in losing 3-1 against Fiorentina.
"We paid for a bit of distraction in the opening stages, when (Giacomo) Bonaventura scored, but I wasn't displeased with our work-rate and attacking desire," Inter coach Andrea Stramaccioni said. "Atalanta played a really good game. They showed their worth, but certainly Inter didn't deserve to lose. This defeat shouldn't dent our morale."
Atalanta are sixth after adding Inter to their list of scalps this season.
"Our objective is to secure safety as soon as possible and until we get there we're going to keep fighting like this, always playing our game," Atalanta coach Stefano Colantuono said. "When we have reached 40 points, then will we start to consider new options.
"When you play against the big clubs it's no good just sitting back and defending, as sooner or later you are going to concede. If you attack, then you can find that positive situation and it happened three times tonight."
Inter had won all their away games this season but were stunned in the ninth minute when Bonaventura headed in Federico Peluso's cross.
Atalanta goalkeeper Andrea Consigli was in fine form to often deny Inter, and Atalanta should have doubled its lead when German Denis astonishingly hoofed the ball over the bar from five meters out in front of an open goal.
Inter were level when Fredy Guarin fired in a missile of a free kick, but Denis put Atalanta back in front in the 60th and then extended their lead seven minutes later with a penalty after Matias Silvestre appeared to have been harshly judged to have brought down Maxi Moralez.
Rodrigo Palacio pulled back a consolation goal late on and Facundo Parra appeared to receive a harsh decision when shown a red card in stoppage time for a shove on Guarin.
In Rome, there were tributes before the match to Gabriele Sandri, the Lazio fan killed by a stray police bullet five years ago. Tensions were high and a Roma fan was stabbed outside the stadium before kickoff Objects, including a Molotov cocktail, were thrown across the heavy police cordon separating the rival fans.
On the pitch, play had to be halted for a few minutes when the floodlights failed. When it did resume, Erik Lamela headed in Francesco Totti's corner for his seventh goal in six games.
Lazio levelled in the 35th with a stunning long-range free kick from Antonio Candreva and the team took the lead when Miroslav Klose slotted in Hernanes' through ball.
Matters got worse for Roma minutes later as Daniele De Rossi became the 13th player to be sent off in nine matches between the two teams. He was dismissed for punching Stefano Mauri in the face.
Lazio extended their lead immediately after the break as Roma's Ivan Piris headed an innocuous-looking long ball straight to Mauri on the edge of the area and the Lazio striker smashed it into the net.
In a dramatic finale, Mauri got a second yellow card for handball. Miralem Pjanic scored moments later with a free kick from 30 meters after spotting Lazio goalkeeper Federico Marchetti off his line.
Roma almost levelled deep in stoppage time after a fantastic pass from Michael Bradley which Pablo Osvaldo fired wide.
"I'm disappointed with De Rossi's actions," Roma coach Zdenek Zeman said. "We talked for two days about those risks, that you didn't need to react or give in to provocation. Of course emotions are high in the derby, but when you feel it too much you pay."
De Rossi apologized to Mauri but has been dropped from the Italy squad to face France.
Milan was jeered off the pitch at halftime and fulltime after losing at home to Fiorentina.
Former Milan midfielder Alberto Aquilani, Borja Valero and Mounir El Hamdaoui scored for Fiorentina, while Milan's Giampaolo Pazzini pulled one back against his old team.
Alexandre Pato blazed a penalty way over the bar when Milan was trailing by just the one goal, after he had been bundled over by Facundo Roncaglia.
There were fears Napoli's game would be postponed with Genoa on lockdown amid flood warnings, following knock-on effects from Hurricane Sandy.
Genoa, who had lost all three games under Luigi Del Neri, twice went in front, through Ciro Immobile and Luca Antonelli. Former Genoa defender Giandomenico Mesto and Edinson Cavani — who scored four midweek — levelled before Marek Hamsik and Lorenzo Insigne turned the match around.
In the lunchtime fixture, Palermo beat fellow strugglers Sampdoria 2-0 with teenager Paulo Dybala notching his first goals in Serie A.
The loss is Sampdoria's seventh successive defeat and threatens to spell the end for coach Ciro Ferrara's brief time in charge.
Elsewhere, Gabriele Angella scored twice to help Udinese draw 2-2 at Chievo Verona. The defender also had a goal wrongly disallowed for offside.
Marco Andreolli had given Chievo the lead and Alberto Paloschi restored their advantage from the spot in his first game back after a lengthy injury layoff.
Chievo's Luciano had thumped an earlier penalty off the crossbar. Danilo was sent off late on after receiving two yellow cards for giving away both penalties.
Torino picked up their first victory in six games with a 1-0 win over Bologna, while Parma drew 0-0 at home to Siena.