The Tuscans raced into a 2-0 first half lead through Angelo and a Damiano Zanon own goal. Mervan Celik and Eugenio Romulo Togni pulled Pescara level, before Innocent Emeghara struck the killer late blow to consign Pescara to an eighth consecutive loss and leave them nine points adrift at the foot of the table.
Cristian Bucchi labelled this game as a "last chance saloon," for his side coming in and it now looks almost certain that his side will be playing second tier football next season
Before the game, both sides laid flowers and Pescara unveiled a plaque dedicated to Piermario Morosini, after the Livorno midfielder tragically collapsed and died during a Serie B game at the Stadio Adriatico one year ago. Pescara have also named their away stand after Morosini.
The hosts were without suspended duo Giuseppe Rizzo and Birkir Bjarnason, with Antonio Balzano, Luciano Zauri, Uros Cosic, Vladimir Weiss and Juan Fernando Quintero making up a lengthy injury list. Celik returned from suspension, but had to settle for a place on the bench.
The visitors dominated the opening quarter of an hour. No-one wanted to tackle Simone Vergassola as he ran towards goal. His shot shaved a coat off the Pescara post as it rolled just wide. Ivan Pelizzoli continued his heroics from last weekend when he saved Claudio Terzi's point blank header. The veteran shotstopper tipped Alessio Sestu's curling 30 yard effort onto the bar, before the Siena man took advantage of a poor Pescara clearance by chesting the ball down and volleying wide.
It came as no surprise when Siena took the lead on 14 minutes. Sestu's shot was blocked and, with Pelizzoli on the ground, Pescara cleared the ball into the path of Angelo, who directed a header into the net from the near post. It was the defender's first goal in Serie A.
Emeghara almost doubled the advantage three minutes later, but he mistimed the flight of the ball and stabbed over.
Pescara showed some life, as Gianluca Caprari sliced a volley wide from Francesco Modesto's cross, before an unmarked Per Kroldrup saw an effort deflect off his shoulder and clawed off the line by Gianluca Pegolo.
Siena doubled their lead on 33 minutes when Zanon turned Sestu's low cross past Pelizzoli and into his own net from six yards. The home side suffered another blow when they lost Gaetano D'Agostino to injury four minutes later.
In first half stoppages, a deflection ricocheted off substitute Celik, but Pegolo ran out to smother before Ferdinando Sforzini could pounce.
Pescara turned the game around in seven second half minutes. First, it was Celik who reduced the arrears on 52 minutes, after his half volley took a deflection off a Siena defender and past Pegolo. The comeback was complete just before the hour. A rooted Pegolo could only watch as Togni curled a 20 yard free kick over the Siena wall and in off the crossbar. A jubilant Togni went into the stands to hug some of the disabled home fans.
The Dolphins had their fins up, as Emmanuel Cascione forced Pegolo into a low save and Togni blazed over from distance, before Pegolo had to fist clear Caprari's curler from outside the box.
Pelizzoli pulled off a couple of quick fire saves, palming clear Angelo's drive and getting fingertips to deny Nicola Pozzi's glancing header from a corner.
In a frantic final ten minutes, Sforzini got in front of Terzi to head wide, before both sides rattled the woodwork within the space of 60 seconds. Pozzi rattled the upright from outside the box, then Giuseppe Sculli struck the post with a rasping drive.
Siena quickly launched a three on two counter attack and got the winner in controversial circumstances. Substitute Salvador Agra looked to be in an offside position as he rolled the ball across for Emeghara to finish at the back post for his seventh goal of the season. Bucchi protested and although his side kept going, it was all in vain as Siena celebrated a massive step towards possible Serie A safety.
Pescara: Pelizzoli; Zanon, Kroldrup, Capuano, Modesto; (Bianchi Arce 76) Togni, D'Agostino; (Celik 37) Caprari, (Abbruscato 85) Cascione, Sculli; Sforzini
Siena: Pegolo; Terzi, Paci, Felipe; Angelo, Vergassola, Della Rocca, Rubin; Rosina, (Agra 67) Sestu; (Pozzi 73) Emeghara (Bolzoni 87)
Goals: Angelo 14 (S) Zanon 33 OG (S) Celik 52 (P) Togni 59 (P) Emeghara 86 (S)