"We have to still improve even if the lads are doing an extraordinary job."
Inter moved level with Juventus and Napoli, two points behind Roma. Fiorentina is three points further back after its first loss of the season.
"The only thing I didn't like was the result," Fiorentina coach Vincenzo Montella said. "It was a great performance with character and just as we thought we had it under control, we conceded two odd goals. I have to take the defeat as well as the performance but we played at a high level against a strong opponent."
Inter had been ordered to keep one end of San Siro the Curva Nord closed as punishment for fan racism during the draw against Juventus.
Fiorentina was missing three key players as new signing Mario Gomez was injured, along with David Pizarro and Juan Cuadrado. Captain Manuel Pasqual added to the Tuscan side's injury problems as he was forced off with a thigh injury 10 minutes from halftime and was replaced by Marcos Alonso.
Mazzarri started with the same team which had won the past three matches, including a 7-0 victory at Sassuolo on Sunday. And it almost took the lead inside the opening minute as Yuto Nagatomo forced a smart save out of Fiorentina goalkeeper Neto.
It was an open match and Inter almost created another goalscoring opportunity in the 11th minute when Hugo Campagnaro put in a tempting cross from the right. But Fiorentina did well to clear and almost took the lead on the counter when the ball came to Giuseppe Rossi who beat a defender and put in an angled drive which Inter goalkeeper Samir Handanovic parried with a deft, one-handed save.
The action was end-to-end and Inter again threatened moments later when it surged forward and outnumbered the Fiorentina defense five to two, but Saphir Taider fired into the sidenetting.
Rossi has been in fantastic form this season and the U.S.-born forward was again at the heart of the action when he set up Joaquin for an effort which Handanovic did well to parry with his legs.
The Italy striker fired Fiorentina in front with a powerful penalty which gave Handanovic no chance as, despite the goalkeeper guessing the right way, he could not prevent it from going into the bottom right corner.
Inter leveled 12 minutes later as Fiorentina failed to deal with a corner. In the goalmouth scramble, the ball came to Cambiasso, who controlled it on his chest before scissor-kicking it in from close range.
Fiorentina had lost its last 12 trips to Inter Milan, with its last victory coming in May, 2000. And it was to again come away from San Siro empty-handed as Jonathan smashed in a screamer under the bar after beating Matias Fernandez to a chipped pass from Ricky Alvarez.
Massimo Ambrosini, who spent 18 years at Inter's bitter city rival AC Milan, almost leveled in stoppage time but Handanovic got down smartly to smother the shot.
