Siena took advantage of Inter's defensive frailties to record a league double over the Nerazzuri with a textbook display of counter attacking, gaining three valuable points for their relegation fight.
Siena made four changes from their defeat at Udinese last week, Massimo Paci, Nicola Belmonte, Alessio Sestu and debutant Innocent Emeghara coming in for Matteo Contini, Neto (who left for Zenit St Petersburg in the week), Reginaldo and Erjon Bogdani, whilst Inter gave debuts to Ezequiel Schelotto and Zdravko Kuzmanovic who came in for Walter Gargano and the injured Gaby Mudingayi.
Inter immediately took control of the match, passing the ball around comfortably and pushing Siena back into their half. The first half-chance came from Antonio Cassano's cleverly taken free kick, nobody tracked Rodrigo Palacio's run and was nearly clear but Gianluca Pegolo got down well to cover the danger.
Pegolo then had his palms stung by a 30 yard thunderbolt from Fredy Guarin, the keeper managing to tip it wide. From the resulting corner, Juan Jesus took an acrobatic overhead kick which Pegolo again had to be alert to divert past the post.
As the match wore on, Siena began to settle into proceedings and were starting to cause the Inter defence problems on the break. Their first real chance came from a short corner routine, Alessandro Rosina exchanging passes with debutatant Innocent Emeghara and firing a vicious left footed effort from the side of the box which Samir Handanovic had to deal with.
In the 21st minute, Siena's counter-attacking enterprise was rewarded with a goal. A quick break caught the Inter centre backs dawdling high up the pitch and Matteo Rubin's low left-wing cross evaded Javier Zanetti and Handanovic and Emeghara was on hand to tap in the easiest of finishes.
Within 90 seconds, Inter were level, Antonio Cassano receiving the ball in the left channel and appearing to play a pass into the centre of the box. This managed to elude the Siena defence and wrong foot Pegolo to creep in at the far post for a bizarre equalising goal.
Just three minutes after Cassano's goal, Siena were back in front. Another quick break got the ball to Rubin down the left hand side, the winger played it to Alessio Sestu on the edge of the box who cut inside on to his right foot and unleashed a magnificent dipping, curling strike that found the net via the crossbar and left Handanovic rooted.
The Inter defence were all at sea and Siena nearly made it 3-1, Emeghara managing to turn in the box but his shot was just wide.
The second half began as the first had ended, with Inter pushing Siena back with some patient passing, but Siena breaking dangerously whenever possession was handed to them.
It was from a break that the Robur grabbed their third goal, Sestu playing a wonderful through ball for Emeghara to run on to, and Cristian Chiviu, beaten all ends up by the pace of the Swiss striker, made a clumsy tackle from behind, taking Emeghara down, giving away a penalty and earning himself a red card. Alessandro Rosina coolly converted from the spot.
Inter continued to dominate possession despite the loss of Chivu, but Siena were more confident than ever on the break and the Nerazzuri defence looked shakier than ever.
Inter toiled for long periods but weren't able to force Pegolo into much work, Fredy Guarin's powerfully struck free kick being one of only a few real chances for Inter.
Inter nearly pulled a goal back after Pegolo felt he had to deal with Guarin's right wing cross, parrying it straight to Cassano, his side footed effort coming back off the underside of the crossbar and then to safety via a Siena defender's boot.
By now, Siena knew that they just needed to frustrate Inter to come away with the victory and they were extremely effective at this, not allowing Inter a sniff at goal despite their dominance of the ball. The only other chance to pull a goal back came in the 85th minute when Rodrigo Palacio managed to elude the entire Siena defence to get on the end of a Guarin free kick, but he completely miscued his volley and it ballooned harmlessly over.
Siena comfortably saw the result out that will do wonders for their confidence and gained three very valuable points for their fight against relegation.
Siena: Pegolo; Belmonte, Paci, Felipe; Angelo, Vergassola, Della Rocca, Rubin; Rosina (Valiani 69), Sestu (Bolzoni 78); Emeghara (Bogdani 82)
Inter: Handanovic; Ranocchia, Chivu, Juan Jesus; Schelotto (Kovacic 46), Kuzmanovic, Zanetti, Nagatomo; Guarin (Gargano 85); Cassano (Rocchi 73), Palacio
Goals: Emeghara 21 (S), Cassano 22 (I), Sestu 25 (S), Rosina pen 55 (S)
Sent off: Chivu 55 (I)