A late Fabio Quagliarella penalty left Inter's Champions League hopes in tatters as Sampdoria came from behind to win 2-1 at San Siro on Monday.
Stefano Pioli's men were hoping to close the gap between themselves and the top three to six points after Napoli's draw with Juventus, but they could not make the most of their superiority and Quagliarella punished them from 12 yards.
Inter were dominant throughout the first half and deservedly took a lead into the break thanks to Danilo D'Ambrosio's deflected effort in the 35th minute.
The visitors rarely troubled Inter at the back, but they were twice denied by the frame of the goal in the first half, with Quagliarella kept out just before the half-hour mark and Bruno Fernandes unlucky soon after.
Sampdoria did restore parity early in the second half, though, as youngster Patrik Schick scored his eighth Serie A goal of the season to continue his encouraging start to life in Italy.
Mauro Icardi was guilty of a diabolical miss 18 minutes from time as he failed to finish off his former team and Sampdoria took full advantage, Quagliarella scoring the winner against Inter for the second time this season.
Inter should have broken the deadlock in the ninth minute.
Ivan Perisic did well to get past Jacopo Sala and cut a pass back towards the penalty spot, but Ever Banega's first-time effort was pushed away by Emiliano Viviano and Antonio Candreva inexplicably failed to bury the rebound.
Quagliarella almost showed them how it should be done in the 24th minute, as he spun and shot from 30 yards only to see his effort crash off the right-hand post.
But Inter finally made the most of their superiority 10 minutes before the break.
Banega clipped an inch-perfect cross in from the right and D'Ambrosio met it in the centre of the area, his shot finding the bottom-right corner via a huge deflection off Bartosz Bereszynski.
Sampdoria should have levelled on the stroke of half-time, though. Fernandes fired a half-volley against the left-hand post before Schick sliced the rebound wide.
The away side looked the sharper of the two at the start of the second half and put themselves back on level terms five minutes after the restart.
Fernandes' left-wing cross was headed onto the post by Matias Silvestre and Schick helped it over the line.
Inter soon ramped the pressure up again and went close just after the hour, Viviano pushing Perisic's header away.
Viviano was helpless in the 72nd minute as Candreva flashed a teasing ball right across the face of goal, but Icardi remarkably sent his first-time effort over from two yards.
And Quagliarella made Inter pay.
Ricky Alvarez's free-kick struck the hand of Marcelo Brozovic in the area with five minutes to go and Quagliarella rifled his penalty right down the middle to secure Sampdoria their first Serie A win at Inter since 1996.