Serse Cosmi's men earn a stunning victory over the disappointing home side, leaving Inter boss Andrea Stramaccioni with plenty of questions to answer.
Inter came out off the blocks running, with 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.
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 goalkeeper 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. Serving as a wake-up call to Andrea Stramaccioni's side, Siena goalkeeper Gianluca Pegolo, minutes after producing a strong save from an Esteban Cambiasso header, parried a venemous Wesley Sneijder strike.
Disaster struck for Stramaccioni's men in the 73rd minute. A neat exchange initiated by Rosina 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.
Inter substitute Philippe Coutinho scuffed his volley into the side netting as the favorites poured forward, desperate to salvage a point, but it was at the other end that Valiani doubled Siena's lead, depositing an Alessio Sestu cross by Handanovic to ice the result.