Inter Milan's new strikers Samuel Eto'o and Diego Milito scored in the first five minutes of a 3-1 win over Napoli in Italy's Serie A.
With previously perfect Sampdoria losing 2-0 at Fiorentina, four-time defending champions Inter jumped into provisional first place.
Juventus, who have won each of their first four games, visit Genoa on Thursday.
Inter lead with 13 points, Juventus and Sampdoria have 12 each, and Fiorentina and Parma are next with 10 apiece.
Freshly promoted Parma beat 10-man Lazio 2-1 with scores from Valeri Bojinov and Nicola Amoruso, while AC Milan were beaten 1-0 at Udinese, with Antonio Di Natale scoring his league-leading seventh goal.
With two losses, Milan already trail Inter by six points.
"The losses don't create a good mood, but I can assure you that the atmosphere in Milan's locker room is good,'' new coach Leonardo said.
"The goal put us in trouble right away and we had trouble finding the target.''
At the San Siro, Eto'o pounced on a loose ball directly in front of the goal two minutes in and Milito collected a pass from Maicon behind the defence and scored three minutes later, although replays showed he was offside.
Lucio made it 3-0 with a header following a corner in the 32nd for his first score with Inter since transferring from Bayern Munich.
Ezequiel Lavezzi pulled one back for Napoli in the 30th.
Inter manager Jose Mourinho was suspended for the match after insulting a referee at the weekend.
With Adrian Mutu sitting out and pondering the $25.4 million he has been ordered to pay Chelsea, his replacement Stevan Jovetic put Fiorentina in front in the 25th.
Sampdoria's defence left Jovetic all alone to head in a cross from Marco Marchionni.
Results & fixtures
Wednesday 23 September Atalanta 0-0 Catania Bari 0-1 Cagliari Bologna 2-0 Livorno Fiorentina 2-0 Sampdoria Inter Milan 3-1 Napoli Lazio 1-2 Parma Palermo 3-3 AS Roma Siena 0-0 Chievo Udinese 1-0 AC MilanThursday 24 September Genoa v JuventusFiorentina forward Alberto Gilardino then headed in a cross from Juan Manuel Vargas in the 66th.
In Udine, defender Alessandro Nesta hit the crossbar with a header in the 34th for Milan's best chance.
Di Natale nearly sealed the match with a shot off the post in the 50th and Milan finished with 10 men after Mathieu Flamini picked up his second yellow card in the 84th.
After two games on the bench, Ronaldinho went on for Alexandre Pato in the second half.
Francesco Totti converted a penalty in the 87th to earn AS Roma a 3-3 draw at Palermo, raising Claudio Ranieri's unbeaten streak to three games since taking over the Giallorossi.
Antonio Conte was fired from his first match as Atalanta's manager, a 0-0 draw against Catania that earned the Bergamo club their first point of the season.