Fiorentina had started their midweek fixture in fourth place and with one eye on moving to within two points of surprise leaders Roma.
But after Giuseppe Rossi had initially silenced the San Siro by putting La Viola 1-0 up from the penalty spot on the hour mark, new life was breathed into the Milan stadium when Cambiasso levelled 12 minutes later.
Inter, who have started the season brightly under new coach Walter Mazzarri, made it four wins from five games when Jonathan hit the winner seven minutes from time. Fiorentina rallied and Massimo Ambrosini came close in the dying minutes from close range only to see Inter goalkeeper Samir Handanovic pull off a superb save.
Heading into the weekend games Roma sit top with a two-point cushion on Inter, third-placed Napoli and Juventus in fourth, with Fiorentina’s first defeat of the season leaving them a further three points off the pace in fifth.