Giuseppe Biava bundled the ball home midway through the first half and Argentine forward Mauro Zarate struck with a lob early in the second period to show that the early season form of last term's relegation candidates was no false dawn.
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The visitors pulled a goal back through ex-Lazio forward Goran Pandev after a couple of rebounds but in-form Brazilian Hernanes sealed victory in style with a minute remaining.
"It was the best Lazio of the season," playmaker Hernanes, who joined the club before the start of the campaign from Sao Paolo, told reporters.
"I've been trying for a bit to have a game like this with a goal and an assist. We're on the right path and won't give up," added Hernanes who played the ball through for Zarate's goal.
A recent wobble by Lazio had pundits suggesting their resurgence was over but a confident performance against the treble winners will have tongues wagging again about whether Edy Reja's Rome-based side can last the pace in the title battle.
The match was Inter's last in Serie A this year ahead of their participation in the Club World Cup and the pressure is back on Benitez after wins against Twente Enschede and Parma had seemed to secured his position after a shaky spell.
Injury-hit Inter, who saw Dejan Stankovic limp off in the first half and were also without suspended top scorer Samuel Eto'o, played youth team defender Felice Natalino at right back while midfielder Sulley Muntari returned from a layoff.
SEASON'S DUCK
They again lacked fluency and the only bright spot was the previously struggling Pandev breaking his season's duck as the boos rang out around the half-full Stadio Olimpico.
Inter's dearth of attacking options without Eto'o and with Diego Milito only just back in training after injury means a move in the January transfer market for a striker is paramount.
"We knew it would be a tough game before the World Club Cup and without our two best forwards but the team battled until the end," Benitez said.
Inter owner Massimo Moratti has said he is willing to delve into his pockets to avoid the European champions from sinking even further but has described reports of a bid for Sampdoria's banished striker Antonio Cassano as "fantasy football".
Italy forward Cassano could become a free agent if an arbitration panel, which was due to meet on Friday but will now convene next week, rules he breached his contract by verbally abusing the Samp president.
