Valtteri Bottas headed Mercedes team-mate Lewis Hamilton in Friday practice at the Tuscan Grand Prix as Lando Norris crashed his McLaren.
Norris put a wheel in the gravel on the exit of Turn Three, Poggio Secco, and spun into the wall. He was unhurt but the incident ended his session.
Bottas was 0.207 seconds ahead of Hamilton in second practice and was quickest in both Friday sessions.
Red Bull's Max Verstappen was third fastest, 0.039secs behind Hamilton.
- Tuscan Grand Prix full practice results
- Tuscan Grand Prix radio & online coverage details
- Vettel to join Aston Martin
Norris' incident led to one red-flag period, and there was a second for a collision between Racing Point's Sergio Perez and Alfa Romeo's Kimi Raikkonen.
Perez was coming out of the pits and did not see Raikkonen on a flying lap on his outside, and as the Finn turned into the first corner his right rear wheel clipped the front right of Perez's car.
Perez has been given a one-place grid penalty by stewards.
Hamilton said he was not driving well and had a lot of improvements to make for the weekend.
"I was just trying to focus on improving my driving," he said. "There are not a huge amount of changes I need to make to the car.
"There are times as a racing driver there is no point changing the car because you are not driving well enough. There are areas I need to make a change but I will do that overnight and hopefully improve it tomorrow.
"I am trying to push it to the limit but there are sections - the last sector I have got that sorted. The first and second sector are a little bit weak for me, particularly the first sector, and there is plenty to come there."
It was a difficult day for Ferrari, celebrating their 1,000th Grand Prix. Charles Leclerc had a spin early in second practice at Turn 12 and ended up 10th fastest, while team-mate Sebastian Vettel was 12th fastest and ground to a halt out on track at the end of the session with an engine problem.
The Ferraris are painted in a darker shade of red, mirroring the colour used by Ferrari's first F1 car in 1950.
After Red Bull's surprisingly uncompetitive performance a week ago in Monza, the competitive order was more as expected at Mugello, which is hosting F1 for the first time.
Verstappen's team-mate Alex Albon was fourth quickest, albeit more than 0.7secs slower than the Dutchman, and ahead of the Renaults of Daniel Ricciardo and Esteban Ocon, Perez and the Alpha Tauri of Pierre Gasly, who won took his maiden F1 victory at the Italian Grand Prix last weekend.
Mugello lived up to its a reputation as a fast and challenging track full of demanding corners. The drivers were taking the famous Arrabbiata right-handers flat out at more than 170mph on their qualifying simulation runs in second practice and it seems as if the fast left-right preceding them may also be flat come qualifying.
Hamilton added: "I love it, it's fast. It's a real challenge. I'm not that great at it yet - still working at it.
"[Arrabbiata] almost feels like Turkey, the double-left hander there, or triple whatever it is. It's incredibly fast through Six, Seven, Eight, Nine. It's pretty intense. It's amazing to experience it.
"It's a very serious track. You don't go lower than fourth gear. It is so quick and there is not a lot of run-off area, particularly through Eight and Nine. The speeds we're going; I'm just praying the tyres hold together and everyone stays safe."
For the first time in this coronavirus-affected season, spectators are being allowed into the grandstands. Mugello has sold 2,800 tickets for each day.
"I'm a little bit frustrated," said Norris. "A small mistake but a big mistake at the end of the day. The car is OK. A little bit of damage. It has definitely set me back a bit.
"The one track I didn't really want to make any big mistakes. I didn't get the high fuel running in. Not ideal but I missed it last weekend in Monza as well and still recovered well. Not perfect, but I don't feel I am too disadvantaged.
"The car felt trickier than we were expecting. We just seem to be struggling with a bit of all-round grip."
- Frankie Boyle's New World Order: Comedians try to make sense of 2020
- Squad Goals: Go behind the scenes with West Ham Women