Inter were 3-0 up within 19 minutes, risked throwing it away against 10-man Fiorentina until Mauro Icardi completed his brace in stoppages for 4-2.
The Nerazzurri were still looking for their first win under Stefano Pioli after a 2-2 Milan Derby draw and 3-2 Europa League defeat at Hapoel Be’er-Sheva. With Gary Medel and Davide Santon injured, Andrea Ranocchia and Ever Banega came in for Jeison Murillo and Joao Mario. The Viola had won their last four meetings with Inter, home and away, including a 4-1 result here last season.
It was a lightning start, as within three minutes Marcelo Brozovic had hit a scorcher into the roof of the net from just inside the box to give Inter the lead.
Moments later Josip Ilicic’s effort was charged down and only a desperate Gonzalo Rodriguez sliding block prevented Antonio Candreva scoring when clear on goal.
Federico Bernardeschi hit a fresh-air shot from 12 yards and Samir Handanovic’s great reaction save on Nikola Kalinic was irrelevant due to an offside flag.
However, Inter doubled their lead when Ivan Perisic found space down that same left flank and Ciprian Tatarusanu only flapped the pass into the path of Candreva for a tap-in.
Handanovic sprinted off his line to parry after Borja Valero went on a slalom through the Inter defence, including a nutmeg on Joao Miranda.
A similar move paid off at the other end, as Icardi cut back inside to commit Gonzalo Rodriguez, dribbled past Davide Astori and slipped the finish beneath Tatarusanu for a simply sublime solo effort.
Inter hadn’t scored three goals in the opening 19 minutes of a Serie A match since January 1990 against Bologna.
Perisic ballooned over despite a Carlos Salcedo error and Handanovic rushed off his line to block Milic, then the goalkeeper palmed a ferocious Ilicic free kick round the near post.
Gonzalo Rodriguez had penalty appeals when Joao Mario kicked his ankle in an attempt to clear, but Kalinic did pull one back when running on to a long ball and getting in front of Danilo D’Ambrosio for the angled drive.
There was controversy in first half stoppages as Icardi was barged over by Gonzalo Rodriguez well outside the area, but the referee opted for a straight red card in an extremely harsh decision.
Despite going down to 10 men, the Viola started the second half aggressively and Handanovic parried an Ilicic snapshot, while at the other end Ever Banega tested Tatarusanu from point-blank range.
Perisic was clear on goal only to thump the outside of the near post from 10 yards, but Miranda needed a goal-line clearance on Kalinic from a free kick.
Soon after, 10-man Fiorentina did get back into the game on 3-2, as Ilicic went on the counter-attack with Inter unbalanced and his angled drive took Handanovic by surprise at the near post, perhaps with a nick off Miranda’s shin.
The Tuscans continued to push, but this left gaps at the back so Perisic’s volley was deflected on to the side-netting. Inter were increasingly pegged back and Handanovic flew to palm a Borja Valero cross off the head of Kalinic.
On one such counter, Inter seemed certain to finish it with Joao Mario, who somehow ballooned over from eight yards with Tatarusanu already going to ground. It was a candidate for miss of the season.
Fiorentina were furious about the fourth Inter goal in stoppages, as Federico Chiesa seemed to be fouled by Ranocchia, but instead Perisic went on the counter with a step-over and his shot was parried into the path of Icardi for the tap-in.
Sent off: Gonzalo Rodriguez 45+1 (F)