Serse Cosmi's men earn a hugely credible victory over the disappointing home side, leaving Andrea Stramaccioni with plenty of questions left to answer
Eager to arrest their poor record at home, where they failed to win at the start of the season, Inter came out of the blocks running, Antonio Cassano presenting an early menace to Siena's backline by backheeling for fellow summer signing Alvaro Pereira to cross into the box, but the ball was well cleared by an alert intervention from the visitors.
Fredy Guarin ballooned a shot from distance high over the bar, while another neat piece of Cassano action led to the former AC Milan forward driving to the byeline and producing a cut-back for the Colombian, who only arrived a second too late for a tap-in to open the scoring.
Despite their early intent however, the Nerazzurri failed to seize the initiative, as Serse Cosmi's men steadily grew into the game. With Alessandro Rosina, Ze Eduardo and Emanuele Calaio looking particularly active, the Robur earned several avenues into the Inter box, with Calaio drawing a reflex parry from keeper Samir Handanovic in the 22nd minute.
Andrea Ranocchia was spared the blushes after he nearly put the ball into his own net from a cross, following a stellar piece of skill on the edge of the six-year box by Ze Eduardo. Siena custodian Gianluca Pegolo, minutes after producing a strong save from an Esteban Cambiasso header, was then diving at full stretch to parry a venomous Wesley Sneijder strike.
The Netherlands international next spurned a glorious opportunity to net the opener at the Giuseppe Meazza stadium. Played in by yet another ingenious through-ball by Cassano in the 32nd minute, Sneijder instead opted to square the ball to Diego Milito, who had been on the periphery of action until then. The Argentine failed to reach his teammate's pass, and yet another Inter attack broke down.
Left to rue his decision not to take his chances on goal, Sneijder drew a block from Pegolo with a shot this time, but stopper Neto was on hand to sweep up the loose ball. Ranocchia next tried his luck from a header, drawing another mighty save from Siena's keeper, but Inter were to made to pay for their profligacy in front of goal, entering the half-time break wondering how they did not manage to score.
The second period began in similar fashion as the Nerazzurri pressed their attacks with increasing desperation. Cassano, a livewire in the first half, fired the first shot following the break. Pegolo however proved equal to the effort, sticking out a strong hand to deny the 30-year-old.
With action dragging on in solid fashion, both coaches turned to their benches in search of the means to earn a result. Ricardo Alvarez came on in place of the underwhelming Guarin for Inter, while Cosmi aimed to introduce more steel to the midfield by introducing Ribair Rodríguez for Gaetano D'Agostino.
Alvarez almost produced an instant impact by crossing for Milito to head narrowly wide of the post, in what was the striker's first attempt on goal all night. The 24-year-old then tried his luck from distance himself in the 61st minute, driving the ball towards goal but failing to test Pegolo.
Disaster was to strike for Stramaccioni's men shortly after, however. A neat exchange initiated by Rosina in the 73rd minute led to Vergassola breaking into the Inter box. The Siena midfielder took a one-two with Ze Eduardo in his stride to bury the ball past Handanovic, silencing the home support and punishing the Nerazzurri's prior wastefulness.
Substitute Philippe Coutinho scuffed his volley into the side netting as Inter poured forward, desperate to salvage a point, but it was at the other end that Calaio nearly doubled Siena's lead, Handanovic forced to palm away the 30-year-old's goal-bound header. It was not to be his night, however, as Valiani deposited an Alessio Sestu cross past the Slovenian with aplomb.
Coutinho was to test Pegolo one last time of the night, but the Tuscan outfit held on to condemn Inter to their second home defeat of the season, to the frustration of the onlooking Meazza crowd.