Former Torino midfielder Alessandro Rosina blasted a penalty wide a minute from time after Matteo Brighi had fouled Francesco Della Rocca.
Brighi had given Torino the lead in the fifth minute but Reginaldo bundled in the equalizer shortly after the half hour.
However, Rolando Bianchi was left completely unmarked to head home Valter Birsa's cross and restore Torino's advantage.
Torino went further ahead in first-half stoppage time when Danilo D'Ambrosio's clearance from his own penalty area fell to Alessio Cerci who raced clear from the halfway line to score.
Siena was back in the game in the 75th when Reginaldo backheeled a cross for Michele Paolucci to fire home.