Lazio had the opportunity to make ground on the top two, with Juventus losing at Roma and Napoli held to a draw by Sampdoria, but the Rome side did
little in a
one-sided game at Stadio Artemio Franchi.
Innocent Emeghara was on target twice for Giuseppe Iachini's side,
while Alessandro Rosina also beat Federico Marchetti in the Lazio goal.
The visitor rallied late, but failed to find a consolation ahead of its UEFA Europa League clash with Borussia
free-kick tipped over the crossbar by Gianluca Pegolo in the 79th minute, and Ederson's effort tipped on to the top of the woodwork by the Siena
goalkeeper a minute later.
Louis Saha then shot wide from 20 metres.