Roma, unrecognisable from the bickering, blunder-prone side who struggled to finish sixth last season, top the table with a maximum 18 points after another outstanding display against the backdrop of a thunderstorm at the Stadio Olimpico.
Serie A champions Juventus rested playmaker Andrea Pirlo but still completed their ninth match against Torino without conceding a goal, a run stretching back to 2002, and extended their unbeaten run in the fixture to 13.
Frenchman Paul Pogba scored the only goal of a niggly encounter amid claims of offside from angry hosts Torino, who have not beaten Juventus since 1996.
Inter Milan lost ground on the leaders when they conceded a late goal to draw 1-1 at homeless Cagliari, Catania beat Chievo 2-0 to clock up their first win of the season and basement club Sassuolo hit back to draw 2-2 with Lazio.
Juventus remained unbeaten this season with 16 points from six games, level with Napoli who won 2-0 at Genoa on Saturday, with Inter fourth on 14.
Ivory Coast forward Gervinho sprang into action in the 17th minute, collecting Francesco Totti's pass and twisting defender Mikael Antonsson one way and the other before firing a low shot past Curci.
Roma surrendered a two-goal lead before losing 3-2 at home to Bologna last season but, under the leadership of new coach Rudi Garcia, there was no chance of a repeat, as Benatia showed by flicking in the third from a corner in the 25th minute.
Ljajic completed the rout with a delightful dinked finished over Curci.
The only goal of the Turin derby came in the 54th minute when Carlos Tevez headed against the crossbar from a corner and Frenchman Pogba reacted quickly to bury the rebound for Juventus.
"They told me that Tevez was offside leading up to the goal, but we must also say that we were unfortunate before that because Ciro Immobile's (first half) tackle on Tevez should have been a sending him," Juventus coach Antonio Conte told reporters.
Conte has made it clear he will rotate the squad this season and, having rested goalkeeper Gianluigi Buffon one week ago, it was 34-year-old Pirlo's turn on Sunday.
The match was predictably intense, with numerous untidy clashes in midfield and few scoring chances.
Inter went ahead when Argentine forward Mauro Icardi nodded in Yuto Nagatomo's cross in the 75th minute only for Radja Nainggolan to equalise for Cagliari with a deflected shot with two minutes left.
Cagliari were again playing in Trieste, on the Slovenian border and around 1,000 kms from their home town, due to a lack of suitable stadiums in Sardinia.
Goals from Jaroslav Plasil and Lucas Castro gave Catania their first win of the season and two goals from German Denis propelled Atalanta to a 2-0 win over Udinese.
Andre Dias and Antonio Candreva scored in a four-minute spell early in the second half to put Lazio ahead at Sassuolo. However, Ezequiel Schelotto replied immediately and Antonio Floro Flores equalised in the 76th minute from a free kick.
Sassuolo, making their top-flight debut, have still to win a match but have drawn their last two games after starting with four defeats in a row.
Promoted Hellas Verona pulled level on 10 points with sixth-placed Lazio, beating Livorno 2-1 after Jorginho scored the winner with a 74th-minute penalty.