Milan Skriniar's own goal looked to have helped Fiorentina to a draw, but Danilo D'Ambrosio secured a third consecutive win for Inter.
Inter's recent revival continued as Danilo D'Ambrosio's late goal secured a 2-1 win over Fiorentina at the San Siro on Tuesday.
Last-gasp wins against Tottenham in the Champions League and Sampdoria in Serie A in the past week had injected life into the Nerazzurri's stuttering start to the season and another three points will further ease the pressure on Luciano Spalletti.
Inter's cause was aided significantly on the stroke of half-time when referee Paolo Silvio Mazzoleni punished Vitor Hugo for handball inside the penalty area after a VAR review, Mauro Icardi opening his league account for the season from the resulting spot-kick.