Stefano Pioli's side have been in fine form over the past month, and are now four points clear in the race for a top-three finish and UEFA Champions League qualification.
Felipe Anderson netted an early opener for Lazio with a powerful header – his eight league goal in nine games – and the advantage was doubled in first-half stoppage time through Antonio Candreva.
Lazio's nearest challengers are now Sampdoria, who climbed two places to fourth courtesy of a 1-0 home win over Inter.
Eder celebrated being called into Antonio Conte's Italy squad for the first time in style with a stunning free-kick midway through the second half which swerved away from Samir Handanovic and went in off the post.
Samp's fourth consecutive win saw them leapfrog Napoli and Fiorentina, who both dropped points.
Napoli needed a late Duvan Zapata header to rescue a 1-1 draw at home to Atalanta, who had earlier seen Alejandro Gomez sent off, while his namesake Mario scored both of Fiorentina's goals in a 2-2 draw at Udinese.
At the summit, Juventus maintained their 14-point lead as Carlos Tevez's fine individual effort earned them a 1-0 home win over Genoa, although the Argentinian did miss a penalty in the second half.
Second-placed Roma triumphed by the same scoreline at struggling Cesena, Daniele De Rossi earning them just a second league win in 11 outings.
Elsewhere, Empoli defeated Sassuolo 3-1 and beleaguered Parma's woes continued as they saw captain Alessandro Lucarelli sent off in a 2-0 home defeat at the hands of Torino, capping a miserable week which saw them declared bankrupt with debts topping £218million.
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