Two goals by Innocent Emeghara and one from Alessandro Rosina gave the Robur a third straight home victory and extended the Biancocelesti's winless run to five games.
Siena missed Brazilian Angelo and gave Emeghara a lone role up front. Lazio were without the injured Stefano Mauri, Cristian Brocchi and Miroslav Klose, while Cristian Ledesma was suspended. Hernanes and Alvaro Gonzalez marshalled a makeshift midfield as Sergio Floccari and Libor Kozak renewed their Europa League partnership in attack.
Lazio dominated the first five minutes of possession, but were caught cold a minute later with Siena's first attack. Emeghara got in front of Michael Ciani to meet Matteo Rubin's cross with a bullet header from six yards, which flew past Federico Marchetti and into the net.
Antonio Candreva drew a full dive from Gianluca Pegolo, only to see his shot finish wide of the post. Kozak then headed down from close range into the shotstopper's gloves. Ciani was booked after a number of infringements and will be suspended for miss next week's game against Pescara.
Things got worse for the Frenchman and for Lazio when Siena doubled their lead midway through the first half. Emeghara's clever backheel found Rubin, whose cross to the near post was stroked in by the waiting Rosina. Once again, Ciani had left his marker and he was soon hauled off for Bruno Pereirinha.
On the half hour, Pegolo palmed out a Candreva shot and Christian Terlizzi blocked Kozak on the follow-up. Shortly after, Felipe caught Kozak in the face with his studs. Not for the first time this season, the Czech striker returned with a white hairnet to protect a nasty head wound.
Just before half time, Abdoulay Konko flashed a ball across the box which was too far in front of an outstretched Kozak. In stoppage time, Terlizzi played with fire as he juggled the ball over Kozak in his own box. The veteran was let off the hook when Kozak handballed.
Candreva ballooned over a direct free kick after the restart. Alessio Sestu tried to play Emeghara in on goal, but Marchetti ran out to smother the ball. The Nigerian-born Swiss frontman was then flagged offside after running onto Terlizzi's through ball.
Francesco Valiani made an instant impact upon coming on. His cushioned header released Emeghara who beat Giuseppe Biava for pace and stabbed the ball under Marchetti for his fourth goal in three games. The forward has been an instant hit since joining on loan from Sochaux in the January transfer window.
Pegolo performed a one handed save to tip over a Hernanes free kick, while substitute Ederson grazed the top of the cross bar with a long range effort. The Brazilian was left all alone in the box after the Siena defence went to sleep, but sent a disappointing header straight at Pegolo.
Siena: Pegolo; Teixeira, Terlizzi, Felipe; Belmonte, Vergassola, Della Rocca, Rubin; Rosina, (Bolzoni 79) Sestu; (Valiani 60) Emeghara (Bogdani 76)
Lazio: Marchetti; Konko, Biava, Ciani,(Pereirinha 32) Dias; Candreva, (Saha 67) Gonzalez, Hernanes, Lulic; Floccari, (Ederson 78) Kozak
Goals: Emeghara 6, 61 (S) Rosina 23 (S)