Veteran Macedonia striker Goran Pandev scored the second against his old club as Inter slipped below AS Roma, who earlier won 2-0 at Udinese after Cengiz Under scored his fourth goal in three games.
Inter ended an eight-match winless run in the league by beating Bologna 2-1 at San Siro last Sunday but if they thought it signalled a change in fortune, they were mistaken.
On the stroke of halftime, Ervin Zukanovic swung in a cross, which Milan Skriniar hooked away. But his clearance ricocheted off Andrea Ranocchia and flew into the net, leaving goalkeeper Samir Handanovic stranded.
Handanovic had previously had a let-off when he misjudged Pandev’s cross and the ball bounced over him, hit the crossbar and fell back into his arms.
Inter, who led the table in early December, fell further behind just before the hour as Diego Laxalt’s shot from the edge of the penalty area was intercepted by Pandev, who controlled it superbly with one touch and placed his shot past Handanovic.
“It wasn’t an especially bad performance, we did what we had to but then we conceded the own goal and that created problems because when you’re in a bad patch, it becomes difficult to react,” said Inter coach Luciano Spalletti.
“In the second half, we tried to take control of the match we couldn’t move the ball around well enough. As soon as something goes against us, we lose our cool.”
Roma, who travel to Ukraine to face Shakhtar Donetsk in the Champions League on Wednesday, scored twice in the last 20 minutes at Udinese to clock up their third successive win.
Under set them on the way, the Turk scoring with a 20-metre shot which swerved away from Albano Bizarri, and Diego Perotti sealed the win after capitalising on a mistake in the last minute.
Roma have 50 points from 25 games although there is a 12-point gap between themselves and second-placed Juventus, who visit Torino on Sunday. Leaders Napoli, a further point ahead, host lowly SPAL.
Chievo ended a 10-match winless run by beating Cagliari 2-1 to move away from the drop zone.
Former Juventus and Sunderland forward Emanuele Giaccherini, on loan from Napoli, curled in a free kick for the Flying Donkeys in the 74th minute and Roberto Inglese finished off a counter-attack for the second two minutes later.
Leonardo Pavoletti reduced the arrears for the Sardinians.
Both teams have 25 points, eight clear of the relegation zone.
(Writing by Brian Homewood in Bern,; Editing by Neville Dalton)