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 as 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.
WHY ALWAYS HIM ?— Bologna FC 1909 (@BolognaFC1909en) February 26, 2023
Orso gives Bologna the lead with less than 15 to play
1-0 (76')#BolognaInter #WeAreOne pic.twitter.com/ZetILrcMDS