Giampaolo Pazzini scored a goal and had his nose broken, as Sampdoria lost the leadership in a controversial 1-1 draw at the Stadio Olimpico.
The visitors had two strong penalty appeals after wo players were sent off.
The Blucerchiati knew they needed a victory to stay top of the table following Inter's 5-0 triumph at Genoa last night. This match was also a return to the Stadio Olimpico for the first time since their Coppa Italia Final defeat on penalties in May.
Lazio were without Simone Del Nero, Stephen Makinwa, Lionel Scaloni and Mourad Meghni, while Samp missed Franco Semioli, Fernando Tissone and the suspended Daniele Gastaldello.
The home side had a strong start with Francelino Matuzalem's angled drive flashing across the face of goal and a Tommaso Rocchi strike smothered. At the other end Antonio Cassano went on a slalom to force Nando Muslera into a one-handed save at the near post.
Cassano set up Mannini for a flick with the outside of his foot that caused some problems for Muslera.
The deadlock was finally broken just shy of half-time with Giampaolo Pazzini's fifth goal of the season. Cassano threaded through for Andrea Poli, the move continued and Padalino looped in a cross for Pazzini's header.
It didn't last long, as three minutes later Lazio equalised. Rocchi flicked the ball over the defence for Matuzalem and at the second touch he poked it past Luca Castellazzi.
In stoppages Cassano's free kick skimmed the crossbar and after the restart Rocchi twice failed to make the most of Stefano Mauri assists.
There was a heavy blow for Samp when Padalino was sent off for a second bookable offence on Kolarov.
Rocchi had penalty appeals for a Marius Stankevicius push, then Castellazzi came flying out to save from Matuzalem as Zarate floated the cross into the six-yard box.
Mauri flashed a shot across the face of goal, but Lazio were also reduced to 10 men as a furious Simone Barone received his second booking for a high tackle on Cassano. The midfielder engaged in a row with Gigi Del Neri as he walked off the pitch.
Muslera flew to palm a Reto Ziegler free kick out from under the bar as it was all Sampdoria now at the Olimpico. With seven minutes to go Cassano's blockbuster smacked against the crossbar with Muslera well beaten.
Pazzini was fortunate to remain conscious when he was accidentally punched in the face by Muslera, visibly breaking his nose. It was a bizarre incident, as the striker seemed to get to the ball first, but rather than a penalty the referee awarded a goal kick.
Sampdoria had another potential penalty shout with Ziegler bundled over in a blatant shoulder-charge by Diakhite! It would be difficult to find a clearer spot-kick, but the referee waved play on.
At the other end Zarate set up Julio Cruz to fire at Castellazzi's legs, then Mauri tested the goalkeeper with a fine header.
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