Hamsik header sends Napoli joint second
The Slovakia midfielder converted Walter Gargano's free-kick after 25 minutes to put his team level on points with Lazio, who visited Juventus in Sunday's main programme of matches.
Fifth-placed Palermo, who lost 1-0 to Napoli on Monday after a goal deep in stoppage-time, picked themselves up with a 3-1 comeback home win over Parma as coach Delio Rossi's bold decision to haul off playmaker Javier Pastore paid dividends.
The visitors, with coach Pasquale Marino absent after his mother's death, took an early lead when the unmarked Alessandro Lucarelli headed in a corner and they were fairly comfortable by half-time with the usually dynamic Pastore having an off day.
The Argentine was replaced by Mauricio Pinilla at the break and the Chilean soon equalised on the rebound. Fabrizio Miccoli then benefited from Palermo's new shape to score before Pajtim Kasami's late header went in off Cristian Zaccardo.
Udinese's Pablo Armero and Antonio Di Natale struck in the second half as their side came from behind to earn a 2-1 home win over Fiorentina.
The visitors had gone ahead after 31 minutes when Mario Santana embarrassed goalkeeper Samir Handanovic from distance.
This weekend's matches were due to be hit by a players' strike after a row over a collective contract with the league but a deal was struck on Thursday to avert industrial action.