AC Milan failed to take full advantage of Inter Milan's draw against Palermo on Saturday as they were held to a draw at home to Napoli and missed the opportunity to leapfrog their city rivals to the top of the Serie A table.
Hugo Campagnaro put a lively Napoli ahead in the 13th minute, but Filippo Inzaghi equalised 12 minutes later.
Napoli started the brighter and deservedly went ahead in the 13th minute.
Ezequiel Lavezzi drew everybody to the near post where Christian Abbiati and Massimo Oddo got in each other's way and ended up in a heap as the Argentinian forward pulled the ball back to his compatriot Campagnaro, who side-footed the it into the empty goal.
Brazilian Ronaldindo was the architect of Milan's leveller as he weaved his magic in the 25th minute, lifting a cross to Inzaghi, who was glued in his usual position underneath the crossbar to head in the equaliser.
Siena breathed new life into their battle to avoid relegation with a hard-fought win over Bologna.
Alberto Malesani's men are now unbeaten in six Serie A games although fourth-bottom Lazio's win at Cagliari means they remain four points from safety.
Argentinian Marcelo Larrondo grabbed the only goal for Siena after 10 minutes as the hosts maintained the momentum gained from last week's astonishing result at Juventus, which saw them come back from three goals down to draw 3-3.
Lazio secured only their third away win of the season to move four points clear of the Serie A relegation zone as Cagliari's woeful run continues.
First-half goals from Tommaso Rocchi and Sergio Floccari did the trick for the capital club, who progressed to 29 points from 29 games as a result.
The hosts, on the other hand, have now claimed just one point from their last five games and dropped to tenth in the standings.
Serie A relegation-battlers Atalanta gave their fight for survival a boost with victory over fellow strugglers Livorno, who are now bottom of the table.
Simone Padoin set the Bergamasco on their way with the opening goal after 13 minutes and further strikes from Javier Chevanton and Adriano Ferreira Pinto early in the second half sealed matters.
Andrea Masiello scored five minutes from time to grab a draw for Bari at home to Parma.
The away side were fortunate to take a 36th-minute lead after Bari's Nicola Belmonte turned the ball into his own net following a cross by Damiano Zenoni.
Parma thought they had done enough to secure their third away win this season but Bari battled back to clinch a deserved point at the San Nicola stadium.
Chievo paid the price for their profligacy in front of goal as Maxi Lopez's penalty ensured Catania snatched a draw in their relegation six-pointer at the Stadio Marc'Antonio Bentegodi.
The hosts wasted a plethora of chances before Sergio Pellissier eventually gave them the lead in the 64th minute but Lopez scored his third goal in as many games to ensure Catania made the long trip back to Sicily with a point.
Catania remain seven points above the drop zone with Chievo only four points better off.
Sampdoria climbed to fifth in Serie A after a narrow win over struggling Juventus in Genoa in Sunday night's late kick-off.
Antonio Cassano scored the only goal of the game in the 77th minute, a breathtaking shot from 32 yards which stunned Juventus goalkeeper Antonio Chimenti, to hand the Blucerchiati three valuable points.
The Samp hero celebrated briefly before asking to come off with a suspected thigh injury.
The result lifts Samp level on 47 points with Palermo, who hold the final Champions League qualifying spot.
Meanwhile, Juventus are two points further adrift in sixth place.