A toothless attacking display from Inter saw them beaten 1-0 by Bologna on Sunday as Riccardo Orsolini scored to give the hosts all three points.
The Nerazzurri had four shots on target without scoring at Stadio Renato Dall'Ara as Napoli's charge towards a first Serie A title in more than two decades received yet another boost.
Musa Barrow saw an early goal disallowed for offside, though it would matter little as Orsolini fired home a 76th-minute winner to clinch victory and put another dent in Inter's already slim title hopes.
With an 18-point deficit to overturn, catching runaway leaders Napoli now looks a hugely improbable prospect for Inter and they may now have to focus on fending off the likes of Roma and Milan for second spot.
Bologna thought they were ahead after 12 minutes through a fine finish from Barrow, but a VAR review ruled the offside Nicolas Dominguez as having blocked Andre Onana's view.
The hosts continued to look the more threatening as Roberto Soriano hit the crossbar, though Lautaro Martinez should have done better when he sent a free header wide of the post.
Clear-cut chances for both teams continued to be few and far between as the game went into the break goalless, but Hakan Calhanoglu missed a golden opportunity shortly after the interval when he blazed over.
Soriano saw a shot blocked, before Edin Dzeko came off the bench to force a decent save out of Lukasz Skorupski.
The deadlock would be broken with 14 minutes to play, though, as Orsolini latched onto Jerdy Schouten's throughball before firing home his fourth goal in five matches to claim an impressive victory.
What does it mean? Drab Inter not matching Napoli's stunning form
The title race looks all but over as Napoli's closest challengers continue to falter while they march on, rattling off eight straight Serie A victories since they were beaten by Inter in early January.
The Nerazzurri looked a far cry from challengers here as they failed to score against Bologna for the first time in eight league meetings.
Inter accumulated just 1.01 xG (expected goals) to Bologna's 1.28 as Skorupski enjoyed a relatively comfortable 90 minutes between the sticks before watching Orsolini secure all three points for his team.
Orsolini the hero
In-form winger Orsolini has now been involved in six of Bologna's last seven Serie A goals as he fired them to a fifth victory in six league games.
The goal capped an impressive display from the 26-year-old, registering three shots and creating three chances as well as scoring the well-taken winner.
Ineffective Lukaku fires a blank
Romelu Lukaku managed just nine touches in the first half while also not creating any chances as he failed to provide much of a focal point for Inter.
Simone Inzaghi saw enough after 64 minutes, deciding to bring on former Manchester City striker Edin Dzeko with Lukaku unable to add to his two Serie A goals this season.
Key Opta Facts:
- Bologna have won four of their past five Serie A games (L1), three of these by just a one-goal margin.
- Inter have failed to score in two consecutive away games in Serie A for their first time since April 2018 (three).
- Orsolini has been involved in 10 goals in a Serie A season for the fourth time with Bologna (seven goals and three assists in the current campaign).
- Inter have not score a single goal with their last 40 shots fired on the road in Serie A.
- The Nerazzurri have conceded at least nine shots in a Serie A first half for only the second time this season, after doing so against Milan last September (13).
What's next?
Inter host Lecce in Serie A next Sunday, while Bologna travel to Torino the following day.
