Their advantage at the top was extended to four points after second-placed Inter Milan were held to a 1-1 draw by Sampdoria in driving rain at the San Siro.
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Samuel Eto'o poked in the lively Coutinho's cross 10 minutes from time, his 15th goal in 13 club games this term, to hand the champions a deserved point in the late game after many chances.
Stefano Guberti had put Sampdoria ahead midway through the second half when Julio Cesar failed to keep out his acrobatic effort after Antonio Cassano had escaped the referee's whistle when robbing Cristian Chivu of the ball.
The downpour was so heavy that Samp's white shirts became almost see-through while Inter coach Rafael Benitez was constantly wiping his glasses as play became difficult.
Wesley Sneijder, who reports say is stalling on a new contract amid interest from the likes of Manchester United, had a rare off day for the treble winners who still lack options from the bench given Diego Milito's injury.
AC Milan can cut Lazio's lead to two points if they win at Napoli without the injured Ronaldinho on Monday.
Sergio Floccari's first-half goal and a fortunate second from Stefano Mauri when the ball went in off his chest helped Lazio to 19 points, their best ever total in the top-flight after eight matches.
Alessandro Matri pulled one back for Cagliari to set up a nervous finish but Edy Reja's group of journeymen, relegation candidates last term, hung on to top spot for another week.
PENALTY SAVED
Lazio's rise to the top of the table seems to have rubbed off on this term's new mascot, Olimpia the eagle, who decided to settle on the Stadio Olimpico roof rather than land on the pitch as instructed during a pre-match routine.
Handlers managed to get him down as the game went on with rivals now plotting how to drag Lazio back down to earth too.
"We suffered a lot in the second half. There was a bit of pressure and fear," Reja, whose side host Serie B's Portogruaro in the Italian Cup on Wednesday, told Sky TV.
"But this team shows great character and determination. Obviously talk of the scudetto is wide of the mark."
Thrilled Lazio fans, who still only filled less than half of the Olimpico, will have even wider smiles this week after faltering city rivals AS Roma drew 0-0 at bottom side Parma and visiting captain Francesco Totti was substituted at halftime.
A mini-revival from fifth-placed Juventus ended when they were also held to a goalless stalemate at Bologna whe
