Lewis Hamilton set the pace for Mercedes in Friday practice at the Italian Grand Prix as his likely replacement Andrea Kimi Antonelli crashed on his debut for the team.
Hamilton headed McLaren's Lando Norris by 0.003 seconds, with Ferrari's Carlos Sainz third in the second session. They were followed by their team-mates Oscar Piastri and Charles Leclerc.
Championship leader Max Verstappen was only 13th fastest after making a mistake on his qualifying simulation run and running wide at Parabolica.
But the Dutchman looked quick on the race-simulation runs later in the session, pretty much neck and neck with title rival Norris.
In the first session, Verstappen was fastest by 0.228 seconds, with a time set on the soft tyre, from Leclerc, who was on the medium tyre. McLaren’s Lando Norris was third ahead of Ferrari’s Carlos Sainz.
Norris, who trails Verstappen by 70 points in the championship, said he expected all four top teams to be in the fight for victory this weekend.
"The car was quick," Norris said. "Oscar was quick. He made a big mistake and lost probably 0.4 or 0.5 seconds so he easily should have been P1. I am quite a long way off, so a bit of work for me to do.
“It’s close between Ferrari, us - Red Bull I’m sure are after P1, and Mercedes as well. Looks like eight cars all looking fast. Should make it exciting.”
A difficult start for Antonelli
Italian Antonelli drove George Russell's car in first practice, as part of his preparations for F1 - he is expected to be named as his team-mate at Mercedes next year in the coming days, following Hamilton's move to Ferrari.
The 18-year-old went fastest on his first flying lap but - in trying to further improve after Hamilton had beaten his time by 0.6secs - he took too much speed into Parabolica and crashed heavily.
He was unhurt but the car was badly damaged and was not ready for Russell until nearly 25 minutes into the second session.
Russell's day was then compromised further when Kevin Magnussen crashed his Haas a few minutes later, leading to a red flag for just over 10 minutes.
But he still managed to end the day sixth fastest after a run on the soft tyres.
Haas' Nico Hulkenberg, RB's Daniel Ricciardo and the Aston Martins of Fernando Alonso and Lance Stroll completed the top 10.
Mercedes F1 team principal Toto Wolff said he believed that the pressures and circumstances of Antonelli's debut might have been too much for him.
"Clearly here, with everything piling up on him in Monza, that is very difficult to cope with. Is that the reason why he put it in the wall? Maybe," Wolff said.
"For him, it feels certainly terrible, but it is part of the development curve. What we see is there is performance, and we have even seen that in the few laps we have seen but what he did, the car couldn't take.
"A strong driver needs to recover from these things, cope with the pressure, but obviously this weekend wasn't easy for him - you still need to compete in F2, you have all these shenanigans around you in Monza, an Italian kid that is being hyped, first time in a Mercedes car.
"That must be a heavy burden but if he wants to be a champion one day he needs to cope with that, and I’ve no doubt that he can and he will."
Antonelli, who is also racing in Formula 2 this weekend, said: "Unfortunately it ended quite quickly. It was quite a big one, around 52Gs, and really sorry to the team and George for making them work afterwards.
"Just a mistake by my side, just pushing a bit too much for the conditions. [I] should have built the run a bit more progressively, but lesson learned for next time.
"Not feeling super-well at the moment. Just going to go back and try to rest and focus for the rest of the weekend. There are still some races to go."
Fellow debutant Franco Colapinto made a solid start for Williams, with the academy driver replacing Logan Sargeant for the rest of the season.
How does form look?
Verstappen had a difficult time last weekend at his home race at Zandvoort, where he was outpaced by Norris as the Briton took one of the most dominant wins of the season.
But McLaren do not seem to have the same advantage over the field at Monza, and the top four teams all look closely matched.
Hamilton said the modifications to the track, which has been resurfaced and had all its kerbs changed, caused problems.
"It’s been a good day," he said. "Generally felt pretty good. The new Tarmac and new kerbs have been a bit of a challenge.
"The car felt good from the start. P1, a lot of graining, people struggling with tyres, including us.
"Then made some good changes and the car was feeling even better in this session. Still some work to do on a few things and the long run, so we can go the distance, that’s the challenge at the moment.”
Verstappen - who was also running with his engine turned down, further hiding his pace - said: "[We] tried quite a few things today and P2 was initially not that great, but then it seemed like the long run was a little bit more competitive.
"But [the newly resurfaced track] seems quite aggressive on tyres for the moment; they are opening up. Quite interesting to see how that will evolve for the race.
"I think it will be quite close between a few cars and hopefully we can be in that mix."
