A last-gasp winner from Lautaro Martinez sealed a breathless win for Inter as the Serie A champions fought back from two goals down to edge visiting Napoli 3-2, while Roma also pulled off a late turnaround to beat Atalanta 2-1.
Inter and Roma sit joint-top of the table on a perfect nine points, while Atalanta are on six points and Napoli have three.
"It was a beautiful and entertaining match to watch, a great advert for Italian football," Inter coach Cristian Chivu told DAZN.
"At 2-2 both teams played an open game without thinking of defending the result, I saw big chances at both ends."
Both goalkeepers were forced into early saves in an end-to-end encounter that remained goalless at halftime after Federico Dimarco's free-kick rattled the Napoli cross bar.
The game exploded in the second half when Matteo Politano guided home a shot in the 50th minute before Rasmus Hojlund's curler doubled the Napoli lead three minutes later.
Inter responded almost immediately when Martinez hammered in his first goal of the season from a Dimarco cross. Substitute Marcus Thuram came agonisingly close to a leveller when he headed off the post.
The France international would not be denied twice, though, and smashed a towering header past Alex Meret in the 82nd minute to level the scores.
Chances fell at both ends before Martinez's penalty box instincts proved decisive in the 91st minute as Napoli failed to clear in a goalmouth scramble and the Argentina striker pounced to settle an extraordinary match.
"I'm happy we managed to fight back, that was missing last year," Martinez told DAZN.
"Every year we are raising the bar and trying to show that on the pitch."
Both sides begin their Champions League campaigns next week, when Inter travel to Real Madrid and Napoli host Arsenal on Tuesday.
DYING MINUTES
In Rome, Mario Hermoso and Matias Soule struck in the dying minutes to continue a perfect start to the season for Roma against their coach Gian Piero Gasperini's former club Atalanta.
Ederson's bullet strike early in the second half came against the run of play but looked enough for a backs-to-the-wall victory as Atalanta goalkeeper Marco Carnesecchi kept out wave after wave of Giallorossi attacks.
Roma scored eight goals in their opening two wins of the season, but looked almost out of ideas after replacement striker Santiago Castro was twice denied by the woodwork and Carnesecchi produced a terrific double save.
But Hermoso was left unmarked and looped a header into the far corner to level the scores in the 90th minute.
The Roma players sprinted back to the centre circle as they looked for a quick restart to push for a winner, and that was exactly what they got when substitute Soule fired a composed finish in at the near post to send the Stadio Olimpico wild.
Roma's first win over Atalanta since March 2022, a run of eight matches, will come as a confidence-booster before their eight-year wait to return to the Champions League ends away to Fenerbahce on Thursday.
Earlier, a late Che Adams strike sealed a 2-1 comeback win for Torino as Fiorentina's nightmare start to the season continued.
Mateo Pellegrino's long-range strike put Fiorentina in front early in the second half, but a pile-driver from Rolando Mandragora – who had only joined Torino from La Viola four days earlier – drew the scores level and Scotland international Adams sealed his side's first points of the season in the 82nd minute with a clever flick.
Fiorentina signed 13 players and appointed new coach Fabio Grosso over the summer after a humbling 15th-place finish last season.
The two-time Italian champions have lost their first three games by an aggregate score of 9-1, and were booed off by their own fans following a performance that left them in last place.