Andrea Stramaccioni's side slumped to a humiliating defeat against Serie A's basement club as strikes from Emeghara, Sestu and Rosina seal Siena's win.
Inter Milan lost to last-place Siena for the second time this season after the Nerazzurri were soundly beaten 3-1 on Sunday.
Andrea Stramaccioni's men never looked comfortable as they produced a pedestrian display that saw the Serie A strugglers come out on top thanks to strikes from Innocent Emeghara, Alessio Sestu and Alessandro Rosina.
The visitors' only reply came from Antonio Cassano, but their misery was compounded by Cristian Chivu's second half sending off in the 55th minute, which put Siena in position to take points off the Nerazzurri yet again.
Stramaccioni’s side once again took the field with a back three and Siena looked to exploit their opponent’s lack of width early on, but January loanee Emeghara failed to make the most of a clean break down the left when he couldn’t pick out Alessandro Rosina in the center.
Inter then tested Siena goalkeeper Gianluca Pegolo, who has had to be on his guard all season, and he was forced into a couple of fine stops as Inter looked to take an early advantage. Siena’s stopper was forced into a flying save by Fredy Guarin after the Colombian let fly from 30-yards and moments later he was called into action once more to beat away Juan Jesus’s acrobatic effort.
A whirlwind five-minute spell then saw the hosts take the initiative as the teams shared three goals in a matter of moments.
Siena broke the deadlock when Matteo Rubin broke down the left for the hosts yet again, this time with no Inter player in the middle able to prevent the ball from falling at the feet of Emeghara, who duly obliged by open the scoring with the simplest of tap-ins.
The hosts’ lead barely lasted a minute though, as Inter new signing Zdravko Kuzmanovic picked out Antonio Cassano on the edge of the area. His low, curled cross evaded Yuto Nagatomo in the center and nestled in the far corner for the Italian’s first goal since October.
The even scorline didn’t last long, though, as Inter was undone on 25 minutes by a moment of magic from Sestu. Once again Inter allowed their opponents space on the left and Rosina was able to pick out Sestu on the edge of the area, where he took a touch before sending a curling effort beyond Samir Handanovic and into the top corner.
The away side looked to claw themselves level before the break, but Kuzmanovic’s deflected effort landed just wide of the post, and Cassano’s appeals for a penalty were waived away as the sides entered the dressing rooms level at the break.
The game swung in Siena’s favor just 10 minutes into the second half when Inter was reduced to 10-men after Chivu's sending off, and the hosts were given the chance to double their lead from the penalty spot.
Despite changing formation to a 4-4-2 at the break, Inter was were caught out by a long ball over the top which put Emeghara clean through. The Swiss international’s surge on goal was halted by Chivu’s rash challenge inside the area, with the Romanian seeing red and Rosina dispatching the penalty into the bottom corner to give his side a commanding grasp on the game.
Siena sat back on their lead and invited 10-man Inter in the final quarter, but despite Guarin going close with a free kick and Cassano rattling the crossbar with an effort in a crowded penalty area, the home side was able to hold on to complete a famous double over the Nerazzurri.