It all looked easy for Torino as they led 3-1 at half-time thanks to goals from Matteo Brighi, Rolando Bianchi and Alessio Cerci, with Reginaldo replying for Siena.
But Michele Paolucci pulled a goal back for the Bianconeri and former Torino midfielder Alessandro Rosina missed a penalty in the 89th minute as the Granata only just held on to a win which lifts them seven points clear of the bottom three, and leaves Siena rock bottom.
Torino took the lead in the fifth minute when Cerci sent Danilo D'Ambrosio towards the goal-line, and he pulled the ball back into the path of Brighi, who accepted the invitation from eight yards out.
Siena's first effort on goal came after half-an-hour when Francesco Valiani's shot was too weak and straight at Jean Francois Gillet in the Torino goal.
Two minutes later, they levelled in contentious circumstances when Reginaldo appeared to use his arm to bundle the ball over the line from a corner.
Torino responded well and Bianchi put them back in front six minutes later with his seventh goal of the season, a header from six yards out as he met a perfect cross from Valter Birsa.
A late assault on the Torino goal saw Massimo Paci's header tipped over the bar well by Gillet as Siena looked to level before the half-time break.
But, from Siena's third corner in a minute, Torino broke and Cerci raced clear, using his pace to leave three defenders in his wake before keeping his composure to round Pegolo and slot the ball in.
It seemed all over, but it was not.
Siena emerged for the second half with their heads held high and Paolucci brought them right back into the game in the 75th minute when he latched onto a deft touch from Reginaldo to beat Gillet.
Torino started to look a bit edgy and Brighi was adjudged to have fouled Paolucci just inside the penalty area a minute from the end.
Rosina stepped up to the spot to deafening boos from his former fans, and they did enough to put him off as he placed his spot-kick wide of the goal, squandering Siena's last chance of the game.