Lazio's win is their fourth in a row in the league and it moves them up above Napoli into the Champions League spots.
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The hosts were in front after just five minutes at the Stadio Olimpico through Lucas Biglia, who crashed home a low first-time volley into the far corner from a high looping clearance.
Lazio continued to dominate and only a string of saves from Viola keeper Neto prevented them from going further in front before the interval.
Milan Badelj squandered a chance to level the scores shortly after half time and soon Lazio had doubled their lead from the penalty spot.
By full time Lazio had a four-goal advantage thanks to a pair of goals in the final 15 minutes from veteran striker Miroslav Klose.
First, the German stooped to head home in the 75th minute before putting the icing on the cake five minutes from time with another close-range finish.
Fiorentina's failure to score means that they have now not found the net in their last four league meetings with Lazio.