Milan: Alejandro DarÃo GÃ³mez scored twice as Atalanta overpowered Lazio 4-1 in Rome while Serie A title rivals Inter Milan crushed promoted Benevento 5-2 on Wednesday.
The two scheduled season-opening matches had been pushed back because of Atalanta and Inter's European campaigns in August.
And the two northern rivals gave a demonstration of their firepower which they hope will deny Juventus a 10th consecutive Serie A title, with the champions settling for a point at Roma on Sunday.
Atalanta and Inter are now both top of the Serie A table alongside Napoli, Hellas Verona, and AC Milan with a maximum of six points from two games.
Juve have four points before they host Napoli on Sunday.
Atalanta were three goals up at half-time with Robin Gosens, Hans Hateboer, and Gomez all scoring, and avoided last season's slip-up where Lazio came back from three goals down to draw 3-3.
"We all knew what had happened last time and we talked about it at half-time," said Netherlands defender Hateboer.
"We knew that when they score a goal they could score others, but when they scored we got the fourth and closed it."
Felipe Caicedo pulled a goal back for Lazio after 57 minutes but Gomez scored his second goal just after the hour mark.
"The squad is the same (as last season) and this is a strength," added Hateboer of his side who finished fourth last campaign.
"But it's too early to talk about the Scudetto."
Simone Inzaghi's Lazio have another tough fixture on Sunday when they host Inter.
"The defeat hurts and burns, but the performance was there," said Inzaghi. "The result does not reflect the match."
After their last-gasp 4-3 win over Fiorentina last weekend Antonio Conte's Inter achieved another big win over Benevento who had shocked Sampdoria 3-2 last time out.
Inter Milan's Romelu Lukaku scored twice, the first within 30 seconds, and Achraf Hakimi impressed on his debut start.
Hakimi, who has spent the past two seasons on loan from Real Madrid at Bundesliga club Borussia Dortmund, unlocked the match with his acceleration down the right finding Lukaku to blast in the opener.
The Morocco international then set up Roberto Gagliardini to volley in after 25 minutes and Lukaku for Belgian's second just before the half-hour.
Hakimi got on the scoresheet three minutes before the break, catching the Benevento defence napping with Argentine Lautaro Martinez making it five with 20 minutes to play.
"I enjoy watching the team play like they did tonight, with incisive attacking moves," said Conte.
"The outright biggest source of satisfaction for a coach is to see that, no matter who's playing, the idea of the game doesn't change.
"We've got more options and we're a team that likes to play, that attacks and that creates lots and lots of chances.
"We've got to work more on our balance because today we also conceded two goals. But it seems that the new arrivals have slotted in rapidly to a tried-and-tested system from last season."
Arthur Vidal and Christian Eriksen also came close to scoring for last season's runners-up, the Dane rattling the crossbar late.
Gianluca Caprari scored for Benevento after 34 and 76 minutes.
"I'm very happy with how I have adapted to the team even if I still have to learn certain mechanisms," said Hakimi.
"Some moves are similar to those I did in Dortmund, I have the skills to play in this team.
"The teammates helped me a lot, and I'm learning quickly.
"The Scudetto? We are working towards this, we have a great team to fight for this goal."
Promoted La Spezia got their first-ever top-flight win, 2-0 against Udinese, despite being a man down for the final half an hour. Andrey Galabinov scored both goals.
"A historic result! With the head, pride and heart," the Ligurian club celebrated on Twitter.