Inter held off Frosinone's second-half fightback to strengthen their position inside Serie A's top four thanks to Monday's (AEST) 3-1 win at Stadio Benito Stirpe.
Goals from Radja Nainggolan and Ivan Perisic before the interval appeared to put Luciano Spalletti's side on course for a routine victory over their relegation-threatened hosts, who remain eight points from safety.
But Francesco Cassata halved the deficit just past the hour with an effort that Inter goalkeeper Samir Handanovic ought to have repelled.
That strike breathed fresh life into the contest as the Nerazzurri were forced to hold on somewhat grimly for the remainder, ultimately securing a success that cements their place in third thanks to Matias Vecino's injury-time goal.
They now sit six points clear of fifth-place Roma, who visit San Siro next weekend.
Frosinone came into the game in possession of the division's worst home record and found themselves behind inside 20 minutes as Danilo D'Ambrosio crossed from the right and Nainggolan steered the ball into the bottom-left corner.
Luca Paganini ought to have done better than guide a free header over from eight yards as the hosts sought a swift response and they were masters of their own downfall in going two behind before the break.
Raman Chibsah inexplicably bear-hugged Milan Skriniar to the floor at a corner and Perisic calmly converted the spot-kick, sending Marco Sportiello the wrong way.
Sportiello saved impressively from Matteo Politano one-on-one as Inter threatened to put the game completely to bed and that stop took on added importance as Cassata's effort from the edge of the box squirmed under Handanovic's right arm.
Camillo Ciano whipped a free-kick narrowly wide and Lorenzo Ariaudo headed straight at Handanovic as Frosinone harnessed their momentum, only for any hopes of salvaging a point to be decisively quashed by Vecino's well-worked goal.
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Nainggolan arrives right on time
Not necessarily best known for his goalscoring prowess, the Belgian midfielder timed his run into the box perfectly to meet D'Ambrosio's delivery, providing Inter the platform to prosper.
Chibsah's cheap foul proves costly
There was no need for Chibsah to take such an aggressive hold of Skriniar, and the two-goal deficit established by Perisic's penalty proved too great for Frosinone to overturn.
Inter face a huge home fixture with fellow top-four hopefuls Roma on Saturday, while Frosinone's fight to stave off relegation continues at Cagliari.