Marek Hamsik gave hosts Napoli an early lead 10 minutes into the game before Paolo Cannavaro and a Davide Astori own goal made it 3-0 as they galloped towards a fifth straight league win.
Inter Milan and AS Roma picked up their first points of the Serie A season on Saturday after sharing an entertaining 0-0 draw at the San Siro.
Midfielder Maxi Moralez scored a controversial goal six minutes from time to give Velez Sarsfield a 1-0 win over Huracan and the Argentine Clausura championship on Sunday.
Cristiano Ronaldo continued his prolific start to the season with four goals, including two penalties, as Real Madrid came from behind to beat Elche 5-1 in La Liga on Tuesday.
Barcelona's lead at the top of La Liga was cut to two points on Saturday as they squandered a two-goal advantage to draw 2-2 at Sevilla while Real Madrid powered their way to a 3-0 home win over Eibar.
Barcelona lost for the second straight La Liga game in a 1-0 home reverse against Celta Vigo but there was no such trouble for Manchester City, who beat 10-man Manchester United 1-0 thanks to Sergio Aguero.
Barcelona followed up last weekend's 3-1 defeat at Real Madrid with another reverse on Saturday, squandering a host of chances to lose 1-0 at home to Celta Vigo and handing the lead in La Liga to their arch rivals.