Hamilton leads Norris in second Italian practice, Antonelli crashes in opening session
The seven-time world champion ended the day at Monza with a best lap of one minute 20.738 seconds, 0.003 faster than McLaren's Lando Norris who won by a hefty margin in the Netherlands last Sunday.
Carlos Sainz was third in an upgraded Ferrari at the team's home track, suggesting another tight battle at the front.
Norris is the closest rival to Red Bull's championship leader Max Verstappen, albeit 70 points behind going into the weekend at the 'Temple of Speed' near Milan.
Verstappen had led practice one with a lap of one minute 21.676 seconds, nearly a second faster than his benchmark at the same stage last year, but dropped to 14th in the later session without completing a qualifying simulation.
Ferrari's Charles Leclerc was second fastest in the first session.
Practice two was temporarily red-flagged when Haas's Kevin Magnussen hit the barriers.
Antonelli, who turned 18 last Sunday, brought the first session to a halt after 10 minutes when the rear of the Mercedes snapped out of line at high-speed Parabolica and the car spun off into the barriers.
Replacing race regular George Russell for the session on a track completely resurfaced from last year, he had made an encouraging start with a clean first lap as the early pacesetter.
"Sorry," he said over the radio before getting out and walking away.
"Kimi, all good," replied team boss Toto Wolff, who later assured the media that the accident would have no bearing on the team's driver decision.
Antonelli went to the medical centre for checks and the team got the car ready for the second session, with Russell sixth fastest after a delayed start.
"It’s unfortunate it ended the way it did. But he (Antonelli) was on the limit, pushing the limit from lap one, that’s always good to see and it’s all part of the learning curve," team spokesman Bradley Lord told Sky Sports television.
Antonelli is expected to be confirmed by Mercedes imminently as the replacement for Hamilton, a five times winner at Monza who is moving to Ferrari at the end of the season.
McLaren's Oscar Piastri ended the day fourth fastest, after the Australian was sixth in session one, with Leclerc fifth.
"For us, it is no different to always," said McLaren principal Andrea Stella after first practice.
"There will be four teams in contention to win the race and practice one has shown some outliers as well. It could be some midfield cars join the party at the top."
Argentine Franco Colapinto made his debut as a Williams race driver, after the team dropped under-performing American Logan Sargeant, and was 17th in both sessions.
Colapinto also lost control at Parabolica late in the first session and went into the gravel, but kept the car going and returned to the pits.
Red Bull's Sergio Perez was ninth and 15th in the sessions.