The Mercedes driver said he had trained hard but had no idea whether he was fit enough for cars which could be four seconds a lap faster than 2016.
"I don't know if I'll be easily fit enough, or will struggle a bit or be super-underneath and need to work harder," the 32-year-old said.
While confident, he said he did not know if Mercedes would remain in front.
The three-time world champion, in an exclusive interview with BBC Radio 5 live, added that he:
*was over losing the title to team-mate Nico Rosberg last year
*looked forward to the challenge of his new partner Valtteri Bottas
*was concerned some aspects of the new rules might not work
*believed Red Bull's form was the one to watch before the start of pre-season testing
*hoped new F1 owners Liberty Media would implement changes to make the sport more exciting'