Struggling Roma slumped to a third away defeat of the season following a poor performance at Siena on Sunday.
Mario Frick's first-half strike was all that separated the two sides while Giallorossi defenders Philippe Mexes and Christian Panucci both saw red in the second half.
The Giallorossi had hoped their league troubles were over following consecutive victories over Atalanta and Bordeaux, but they were upended in Tuscany for the second season in a row.
Siena coach Marco Giampaolo decided to leave striker Emanuele Calaio on the bench with Frick lining up alongside Massimo Maccarone.
For the visitors, Mirko Vucinic failed to pass a late fitness test so French youngster Jeremy Menez started on the left with Julio Baptista playing as a lone striker.
It took 20 minutes for the first real chance of the game to arrive as both sides canceled each other out in midfield.
Maccarone did well to break free from Mexes and fire a right-footed effort just wide from the edge of the area.
Five minutes later and Roma could have taken the lead through Rodrigo Taddei after he was played in by Simone Perrotta.
Taddei's powerful effort was deflected out for a corner by the feet of Siena goalkeeper, and former Giallorosso, Gianluca Curci.
A minute later and Perrotta was just a second too slow as he slid in looking to tap-in a low Taddei cross from the right. The Robur defense cleared the danger much to the home crowd's relief.
In the 39th minute Daniele Galloppa picked out Mario Frick with a good long pass, but the striker fired just wide with a volleyed effort from the edge of the area.
It was Frick who eventually found a breakthrough a minute into first-half stoppage time.
Robur playmaker Houssine Kharja opened up Maccarone who crossed low for Frick from the right. The Lichtenstein striker got to the ball before Mexes could react and slotted home past Giallorossi goalkeeper Doni.
Roma looked to get back in the game after the restart but it was Maccarone that had the first chance of the second half.
The former Middlesbrough man cut in from the left before firing a rocket from the edge of the area which Doni managed to just push round for a corner.
In the 51st minute Alberto Aquilani's cross was met by the head of Perrotta but the midfielder's effort ended up wide to the left of Curci.
Two minutes later Daniele De Rossi tried his luck with a shot from 35 yards. His powerful left-footed strike was deflected out for a corner by Daniele Ficagna.
In the 66th minute Roma's afternoon worsened as Mexes was shown his second yellow card for dissent.
Less than 10 minutes later and Spalletti was left with just nine men on the field as veteran defender Panucci was also shown a second yellow card for bringing down Robur substitute Abdelkader Ghezzal on the edge of the area.
The hosts could have doubled their lead in stoppage time as Ghezzal picked out Houssine Kharja on the left side of the penalty area but he blasted over from a good position.
