ROME -- Real Madrid castoff Jose Maria Callejon scored in the first half and Belgium international Dries Mertens added a late goal as Napoli handed Serie A leader Juventus only its second loss of the season with a 2-0 victory Sunday.
Still, Juventus has an 11-point lead over second-place Roma and is 17 points ahead of Napoli with seven rounds remaining.
In the 37th minute at the San Paolo stadium, Callejon beat Juve midfielder Kwadwo Asamoah to redirect a cross from Lorenzo Insigne from the edge of the box for his 12th goal since transferring from Madrid in the off-season.
Mertens sealed the result after collecting a long pass from Goran Pandev in the 81st and muscling off Claudio Marchisio before beating goalkeeper Gianluigi Buffon with a long bouncing shot.
Napoli is still hoping to edge Roma for second place and gain the last direct Champions League berth.
"This is a very important win, because it keeps us close to Roma," Insigne said. "We're going to fight until the end."
Juventus, which was without suspended Serie A scoring leader Carlos Tevez, struggled to create chances. The Turin power's only other loss was a 4-2 defeat at Fiorentina in October.
Earlier, Mattia Destro's early goal was key and Roma was fortunate when a penalty kick for relegation-threatened Sassuolo was waved off in a 2-0 win.
Roma faces Parma on Wednesday in the resumption of a match from February that was stopped after eight minutes due to a waterlogged pitch.
"No other squads will be playing," Roma coach Rudi Garcia said. "We've got to take advantage of that."
Elsewhere Sunday, fourth-place Fiorentina was held to a 0-0 draw at Sampdoria.
Fiorentina winger Juan Vargas banged a free kick off the corner of the goal frame in the 78th and Sampdoria coach Sinisa Mihajlovic was sent off for protesting in the final minutes, grabbing an official's arm as he walked off the pitch.
Parma remained one point behind Inter Milan in the race for fifth and the final Europa League berth after a 3-2 loss at Lazio.
Antonio Candreva scored the winner for Lazio in added time of a match marked by poor defending on both sides.
After Lazio's Senad Lulic and Parma's Jonathan Biabiany traded goals in the first half, Miroslav Klose restored Lazio's lead in the 67th and then Lazio goalkeeper Federico Marchetti let the ball trickle in between his legs in the 81st for an own goal.
Luca Toni scored twice after Marco Donadel's opener in Hellas Verona's 3-0 win over Genoa and Alessio Cerci set up one goal for Omar El Kaddouri and scored another in Torino's 2-1 win over Cagliari.
In Naples, only Buffon kept the hosts from scoring early on.
Buffon stopped a close-range attempt from Callejon seven minutes in, Hamsik had a goal annulled for offside seconds later and Buffon tipped an attempt from Hamsik over the bar in the 10th.
Buffon also blocked a powerful effort from Callejon in the 29th then stopped a dangerous free kick from Algeria fullback Faouzi Ghoulam in the 58th.
Napoli was helped by vocal crowd support.
"As always, the fans gave us a big hand," Insigne said.
In Reggio Emilia, Destro had an easy time finding the target from the centre of the area in the 16th after getting set up by Belgium midfielder Radja Nainggolan, who stripped the ball from a defender just outside the area.
Competing for a spot on Italy's World Cup squad, it was the fourth consecutive match in which Destro scored, and it was his 10th goal since beginning his injury-delayed season in December.
Sassuolo thought it had a chance to equalize in the 35th when referee Nicola Rizzoli -- Italy's World Cup representative -- appeared to change his mind several times before denying the hosts a penalty for minor contact between Roma defender Mehdi Benatia and Nicola Sansone.
Roma substitute Michel Bastos sealed the victory with a goal six minutes into added time after getting set up by Roma's 37-year-old captain Francesco Totti, who replaced Destro in the 79th.
Playing its first season in the top division, Sassuolo remained one point above last-place Catania, which visits Udinese on Monday.