Lazio's hopes of remaining in the Serie A title race were damaged by a vibrant and adenturous Siena in a 4-0 thrashing in Tuscany on Saturday.
Mattia Destro scored a brace for the hosts and Emanuele Calaio added two penalties while the visitors had goalkeeper Albano Bizzarri sent off at the end of the first half.
The victory moved Siena up to mid-table while fourth-placed Lazio could drop to seven points off the leaders if either AC Milan or Juventus win on Sunday.
The Romans could also find themselves coming under pressure from fifth-placed Inter Milan if the nerazzurri beat Parma later on Saturday.
This match was over by the break after Siena ripped Lazio apart on the counter-attack.
Brazilian Angelo had a bright start on the right and was involved in setting up chances for Destro and Alessandro Gazzi, both of whom shot wide.
Lazio didn't heed the warning and fell behind when Destro surged forward from midfield and shot across Bizzarri on 11 minutes, although centre-back Giuseppe Biava was guilty of leaving a gaping hole for the forward to attack.
Djibril Cisse tried a speculative bicycle kick that went hopelessly high in a rare Lazio foray.
Caliao shot wide after another swift Siena counter as he was set up by Franco Brienza.
Lazio started to come back into the match and Albania midfielder Loik Cana brought a smart save out of goalkeeper Gianluca Pegolo.
Cisse then set up Giuseppe Sculli who seemed to lose his balance at the back post and sliced his shot woefully off target.
Again Lazio were made to pay as Brienza went down after trying to squeeze through a Marius Stankevicius and Lionel Scaloni sandwich.
Calaio scored from spot on 35 minutes before Pegolo tipped headers from Cisse and Sculli around the post.
But on the stroke of half-time the game was effectively ended as Destro ran between Stankevicius and Senad Lulic, the pair letting him pass all too easily, perhaps wanting to avoid giving away another penalty.
Destro dinked the ball over Bizzarri, who appeared to clip him with his foot.
The ball hit the post and Destro jumped up to slot home the loose ball except that the referee, perhaps somewhat prematurely, had already blown his whistle to give a penalty.
Bizzarri was dismissed and Calaio sent his second penalty into the same spot as his first, with substitute goalkeeper Juan Pablo Carrizo's first task being to pick the ball out of the net.
Destro rounded off the scoring nine minutes from time when he headed home Cristiano Del Grosso's cross.