Juventus' 22-match unbeaten run in Serie A came to an end on Sunday while Roma's 2-0 win at Sassuolo was shrouded in controversy.
Defending champions Juve went into their match on the back of seven straight wins in the league but had no answers to Napoli in a 2-0 defeat.
Jose Callejon gave the home side a 37th-minute lead, despite appearing to be offside, rounding off an impressive first-half performance.
Juve goalkeeper Gianluigi Buffon was in inspired form, making a string of saves, but he could not prevent substitute Dries Mertens doubling Napoli's lead with nine minutes to go.
Defeat for Juve - only their second in Serie A in 2013-14 - means their lead at the top of the table stands at 11 points after Roma won 2-0 earlier in the day at Sassuolo, but Antonio Conte's men remain very much in line to secure a third successive title.
Referee Nicola Rizzoli won the headlines at Sassuolo after awarding the hosts a penalty – only to then revoke the decision.
Rizzoli, who will represent Italy at the FIFA World Cup, saw Roma take the lead in the 16th minute as Mattia Destro made it four goals in as many games.
He then awarded the controversial spot-kick in the 36th minute when Nicola Sansone went down under a challenge from Roma's Mehdi Benatia.
The challenge appeared innocuous but the official gave a penalty and, despite ongoing protests, the ball was on the penalty spot before Rizzoli changed his mind.
Benatia later claimed that the goal-line assistant referee had told Rizzoli to award a spot-kick but, after discussions with Sansone, the man in charge decided against it.
Protests continued at half-time and relegation-threatened Sassuolo conceded a second when Michel Bastos scored late on.
Antonio Candreva's stoppage-time winner spared Lazio's blushes in a 3-2 home win over Parma.
With fans continuing to protest at Lazio, another sparse crowd at Stadio Olimpico witnessed a shambolic own goal with just nine minutes left.
Lazio defender Mickael Caini – a first-half substitute – intercepted a cross destined for Parma's Antonio Cassano at the back post.
Yet Caini let the ball bounce before he and goalkeeper Federico Marchetti looked at each other, with the ball agonisingly going through the latter's legs.
That levelled proceedings at 2-2 but Candreva struck late for Lazio.
Other results on Sunday saw Luca Toni strike twice as 10-man Verona beat Genoa 3-0, while Torino edged Cagliari 2-1 and Sampdoria and Fiorentina played out a goalless draw.
|Coach: E. Di Francesco|
for D. Biondini 62'
for S. Floccari 75'
for D. Berardi 81'
|Champions League Final: Luis Enrique worried about Juve, Italian fan nears end of Berlin bike ride|
|Essien joins Panathinaikos|
|Blatter in profile: A look back on 17 years at the top of world football|
|Demichelis set for international retirement after Copa|
|Football world reacts to Blatter resignation|
|Blackpool turn to McDonald|
|Blatter resignation: How the FIFA scandal unfolded|
|Ancelotti rejects Milan approach|
|Platini: Blatter resignation ´the right decision´|
|Swiss Attorney General: Blatter not under investigation|
|Blatter resignation can inspire FIFA change - Scala|
|Mohsni faces lengthy SFA ban|
|Sepp Blatter: Resignation speech in full|
|Blatter to resign as FIFA president|
|Adkins named Sheffield United manager|
|Juve talks ongoing - Witsel|
|Costa has no intention of leaving Chelsea|
|Ligue 1: Paris Saint-Germain season review - Blanc´s men make history|
|Luis Enrique ´worried´ by Juve|
|Mourinho: Costa happy at Chelsea|
|Mourinho hits out at ´unacceptable´ Loftus-Cheek display|
|Inter an ´option´ for Richards|
|Messi: Off-field relationship with Neymar and Suarez key|
|Remy rifle seals Sydney win for Chelsea|
|Messi could replicate Bilbao goal - Luis Enrique|
|Liverpool mauling a boost in transfer market, says Stoke chairman Coates|
|Adkins: Football can learn a lot from rugby|
|Celtic swoop for City defender Boyata|
|Xavi out to sign off in style at Barca|
|Criscito and Poli leave Italy squad|
|Lacazette plays down ´unfounded´ PSG rumours|
|Lloris: No buy-out clause in Tottenham contract|
|Newcastle released Taylor and Gutierrez in same phone call|
|Alba signs new Barcelona deal|
|Benteke coy on Villa future|
|FIFA denies Valcke involvement in $10million payment|
|Brazil selection never influenced, says technical director|
|Pogba doing ´great things´ and will get better, says Deschamps|
|Bundesliga: Borussia Dortmund season review - Klopp endures rocky farewell|
|Adler ´aged three years´, Labbadia ´extremely proud´ of Hamburg|
|Colombia squad settles in Argentina|
|Balotelli ´a target´ because of Liverpool form - Richards|
|Bosnich: If I´d listened to Terry I would have avoided drugs|
|Luiz Gustavo ruled out of Brazil´s Copa America squad|
|Mexico confirm Copa squad|
|Pogba ´very happy´ at Juve|
|Pogba can be Europe´s greatest - Pirlo|
|Pinedo stays in Argentina, Bolivia captain Raldes in Copa America squad|
|Quinteros cuts Banguera, Hidalgo for Chile showpiece|
|Injured Gonzalez misses Copa America, Venezuela finalise youthful squad|
|Champions League Final: Buffon plays down Juve hopes, Iniesta doubtful|
|Karlsruher 1 Hamburg 2 (2-3 agg): Muller strike ensures survival|
|CONCACAF official Sanz banned by FIFA|
|Gourvennec shuns admirers with Guingamp extension|
|Pastore signs PSG extension|
|Morel swaps Marseille for Lyon|
|Watford sign Austria defender Prodl|
|Monaco confirm Berbatov departure|
|Peru v Mexico: Herrera confident as Copa preparations continue|
|Injury rules Dost out of Netherlands friendlies|
|Koeman must strengthen attack for Europa League challenge - McMenemy|
|Wilshere apologises for Spurs taunts|
|Marchisio: No fear ahead of Barcelona clash|
|Klopp to take break after leaving Dortmund|
|Champions League win would be ´more than extraordinary´ - Allegri|
|Charity calls for education after Leicester ´racist orgy´ claims|
|Taylor agrees Swansea deal|
|Sevilla snare Krohn-Dehli|
|Coloccini hails Gutierrez after ´perfect´ Newcastle finale|
|Benitez excited by Madrid opportunity|
|Sampaoli confirms final Chile squad for Copa America|
|Clement ready to go it alone at Derby|
|Mihajlovic confirms Sampdoria exit|
|Brentford turn to Dijkhuizen|
|Usami, Moto to lead Japan qualifying threat|
|Vieira ´the best bet´ for Newcastle - Bernard|
|´We´ll see what happens with my future´ - De Gea|
|Klinsmann: Guzan will start at Gold Cup|
|Alves wants Barca extension not PSG - Blanc|
|Derby appoint Paul Clement as head coach|
|Karlsruher ´can only win´ - Kauczinski|
|Ronaldinho causes stir with disallowed goal|
|Szczesny plans Arsenal stay|
|Pioli: Lazio deserve Champions League qualification|
|Pablo Aimar makes River Plate return|
|MLS Review: Seattle leave it late against Red Bulls|
|Wenger: Arsenal won´t spend big|
|Police to investigate Rangers-Motherwell fight|
|´I will definitely stay´ at City - Toure|
|Champagne backs Blatter to reform FIFA|
|Honduras 2 El Salvador 0: Penalty, goal and red card to Lozano|
|Mancini wants Icardi to stay|
|Sampdoria set to speak on Mihajlovic future|