Spaniard Marc Marquez escaped serious injury after crashing at 175mph in practice as world champion Jorge Lorenzo topped the timesheets in Italy.
Marquez, 20, lost control of his Repsol Honda at the end of Mugello's long straight and slid down the grass verge.
His team said on Twitter: "Bad accident for Marc, thankfully he is conscious in the medical centre and seems to be ok."
British rider Bradley Smith, 22, also crashed heavily and broke a bone in his wrist but will race on Sunday.
Marquez has not yet decided whether he will be able to race this weekend.
He said: "Just after the hill on the straight, I grabbed for the brakes but I lost the front. I tried to save it but the bike was pulling me towards the wall so I jumped from the bike to avoid hitting it.
"I hit my chin hard and strained my neck quite badly, I also have some bruising to my right shoulder, arm and leg but other than that I'm OK.
"Thanks to all the medical team that were quickly on the scene to assist me. Now I need to rest and see how I feel later tonight and in the morning."
Marquez's Repsol Honda team later confirmed that he lost control of the bike at 198mph and came off at 173mph.
Smith, in his first season in MotoGP, crashed at turn seven, sustaining a scaphoid fracture in his left wrist and a wound to a finger on his left hand, but has been passed fit to ride. He will have a skin graft next week.
Tech 3 Yamaha boss Herve Poncharal said: "What happened unfortunately can be part of the learning experience in MotoGP. Fortunately he has no major injuries because he landed heavily but the little finger on his left hand was quite badly damaged."
Mugello's straight is one of the longest on the MotoGP calendar, with Marquez setting the fastest speed through the speed trap of 342kmh (212 mph).
Marquez - who is second in the world championship - was fastest in the morning session and ended the afternoon down in 14th.
Lorenzo's Yamaha team-mate Valentino Rossi was second in the afternoon session ahead of Briton Cal Crutchlow.
Combined Mugello practice times:
1. Jorge Lorenzo (Spa) Yamaha 1'48.375
2. Valentino Rossi (Ita) Yamaha 1'48.409
3. Cal Crutchlow (GB) Tech 3 Yamaha 1'48.672
4. Nicky Hayden (US) Ducati 1'49.377
5. Dani Pedrosa (Spa) Repsol Honda 1'49.383
6. Andrea Iannone (Ita) Pramac Ducati 1'49.467
7. Andrea Dovizioso (Ita) Ducati 1'49.543
8. Stefan Bradl (Ger) LCR Honda 1'49.595
9. Randy De Puniet (Fra) Aspar ART 1'49.599
10. Michele Pirro (Ita) Ducati 1'49.649