Inter's 11-game winning streak in Serie A came to an end as Samir Handanovic's first-half own goal helped Napoli seal a 1-1 draw.
Antonio Conte's side are still red-hot favourites to win a first league title since the 2009-10 season, though the gap to neighbours Milan is down to nine points with seven games remaining after their city rivals overcame Genoa earlier in the day.
Inter hit the woodwork twice in the first half, yet they went in at the interval behind after Handanovic had fumbled a cross into his own net.
Christian Eriksen gave Inter hope of securing consecutive away league wins over Napoli for the first time since 1997, but Gennaro Gattuso's hosts held firm to move within two points of fourth-placed Juventus.
After a tepid start, Inter went agonisingly close to going ahead shortly before the half-hour mark when Romelu Lukaku diverted Marcelo Brozovic's strike onto the crossbar from six yards.
Napoli then broke the deadlock nine minutes before the interval after some slapstick goalkeeping from Handanovic.
Lorenzo Insigne's cross should have been comfortably dealt with by the Inter keeper, yet the ball conspired to end up in the back of the net after he collided with Stefan de Vrij when trying to claim it.
Lukaku hit the woodwork again before the interval, steering Eriksen's free-kick onto Alex Meret's right-hand post, as Inter finished the first half strongly.
Inter deservedly pulled level 10 minutes into the second half when Eriksen rifled in just his second Serie A goal for the club from 20 yards.
Matteo Politano struck the crossbar with a thunderous strike late on, while Inter had a nervous wait when referee Daniele Doveri reviewed a De Vrij tackle in the penalty area on the pitchside monitor, the official ultimately deciding the Dutchman had legally won the ball.
What does it mean? Nerazzurri still in pole position
Inter's lead over Milan was cut by two points this weekend, but Conte and his players remain firm favourites to get their hands on the Serie A trophy.
While they hit the woodwork twice at the Stadio Diego Armando Maradona, Inter will probably be happy to head back to the capital with a point given Napoli had more shots (12 to 10) and enjoyed 56.6 per cent of the possession.
Eriksen finds his range
Eriksen's Inter career looked to be over after he was told he was free to leave in the mid-season transfer window, but the Denmark international has fought his way back into Conte's plans. His strike to seal a point was his 24th from outside the area in the top five European leagues since he joined Tottenham in 2013-14, with only Lionel Messi (59) scoring more in that period.
Handanovic almost costs Inter
Handanovic made an almighty mess of what should have been a routine take from Insigne's cross. Since he joined Inter ahead of the 2012-13 season, no goalkeeper in the top five European leagues has scored more own goals (four, level with Lyon's Anthony Lopes).
Inter are in Serie A action again on Wednesday when they travel to Spezia, while Napoli host Lazio in the Italian top-flight a day later.