Juventus sit level on points with Napoli at the top of Serie A on Saturday after a thrilling 4-3 win with Inter Milan as both teams continued their perfect starts to the new season with the Champions League set to begin.

A stunning strike in stoppage time of the 'Derby of Italy' from teenage substitute Vasilije Adzic put Juve on nine points from their first three matches, behind on goal difference from Napoli who cruised to a 3-1 win at Fiorentina with new boy Rasmus Hojlund on the scoresheet.

Midfielder Adzic, who made his debut for Montenegro in June, had come on for Teun Koopmeiners not long before Khephren Thuram levelled the scores at three each in the 83rd minute and let fly with a rocket of a shot which squirmed through Inter goalkeeper Yann Sommer's fingers.

"What can I say, it's a dream to play and score such a hugely important goal for Juve," said Adzic.

"I can only thank the coach and the staff for showing faith in me and bringing me on in such a big game."

Adzic's strike overshadowed the contribution from France's Thuram brothers who netted for each team.

Marcus Thuram headed Inter deservedly 3-2 ahead in the 76th minute after Hakan Calhanoglu twice drew the away team level with superb long-range strikes of his own.

Under the gaze of their iconic father Lilian Thuram, who watched on from the Allianz Stadium stands, Khephren Thuram then nodded Juve level and set the stage for Adzic's powerful strike, which have the Turin giants a huge boost ahead of Tuesday's visit of Borussia Dortmund.

"It was a crazy match, loads of goals and end-to-end, and we're really happy to get a win against such a good team that has really great players," said Juventus coach Igor Tudor.

Inter are stuck on three points after a second straight defeat under novice coach Cristian Chivu, who replaced Simone Inzaghi in the summer even though his only previous elite coaching experience was a handful of matches in charge of Parma last season.

Chivu's team spent large chunks of the match dictating the play in what is traditionally the biggest fixture of Inter's season -- even more important than the Milan derby.

"You have to look at the performance and the fact that we came here and played with personality, try to win the match and not settle for any other result," said Chivu, whose team next travel to Ajax.

- Hojlund strikes -

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Rasmus Hojlund celebrates scoring on his Napoli debut / © AFP

Napoli warmed up for their clash at Manchester City on Thursday with a powerful display in Florence which strengthened their status as the team to beat in Italy.

The champions were two goals ahead within 14 minutes in Florence, with Kevin De Bruyne smashing home a penalty and Hojlund expertly guiding home from Leonardo Spinazzola's pass on his debut.

Hojlund looked a completely different player to the one who flopped at Manchester United as Napoli threatened to run riot amid the jeers of an unhappy home crowd.

And Napoli were assured of the win five minutes after half-time when Andre-Frank Anguissa, who won the penalty De Bruyne scored ahead of his return to Manchester, pushed a flick-on at a corner back into the path of Beukema who tapped in from close range.

Luca Ranieri salvaged some pride for Fiorentina in the 79th minute, too late for the hosts to mount a comeback.

"It's a very important win today, we won the first three now and want to continue like this," said Hojlund to Sky.

"We have a very important game in the Champions League coming up, I am very happy to be with this incredible team."

Roma can join Napoli and Juve on nine points with a win over Torino on Sunday, while promoted Cremonese and Jamie Vardy could keep pace against Verona on Monday.