Angelo and a Damiano Zanon own goal gave the Bianconeri a two-goal cushion at the interval, but Pescara launched a spirited fightback in the second half to level through goal from Mervan Celik and Eugenio Togni.
However, Siena regained the lead of a pulsating relegation battle four minutes to spare with Innocent Emeghera taking them out of the relegation zone and leaving their hosts nine points in their wake.
Siena began brightly with Simone Vergassola going close to giving them the lead within 10 minutes before Claudio Terzi's hit was saved by Ivan Pelizzoli.
Pelizzoli turned Alessio Sestu's curling effort onto the crossbar before the same Siena player went close again during a furious start to the game for the Tuscan side.
Their pressure paid off in the 15th minute when Angelo headed in after Sestu's third shot in as many minutes was blocked.
Emeghara should have made it two shortly afterwards, but he sent his shot over the crossbar from a good position.
Pescara had a good spell of pressure of their own before Siena doubled their lead in the 33rd minute when Sestu's cross was turned past his own goalkeeper by Zanon.
The second half started in a similar vein with Emeghara and Sestu wasting chances before Celik grabbed a lifeline for the hosts.
Giuseppe Sculli's shot was saved by Gianluca Pegolo, but the ball fell out to Celik, whose shot deflected in off Felipe.
The goal transformed the game and Pescara levelled just seven minutes later with a fine free-kick from Togni which grazed the underside of the crossbar on its way past Pegolo.
Suddenly, Pescara were on top and Pegolo did well to deny Emmanuel Cascione.
After withstanding the pressure, Siena began to attack again with Angelo and substitute Nicola Pozzi both testing Pelizzoli.
Pozzi then hit the crossbar with Sculli hitting the post at the other end as the game swung from end to end with both sides searching for a winner.
And it was Siena who found it in the 86th minute with Emeghara completing a counter attack following a Pescara as the home side committed too many men forward.