JOIN USLewis Hamilton admitted that two weeks after claiming his sixth drivers' world title he was not sure if he can mount a serious bid in Sunday's Brazilian Grand Prix where he won from pole position last year.
The 34-year-old Briton, who revealed this week that he is considering his future in Formula One beyond the end of his current Mercedes contract next year, wound up fifth after Friday's second session.
Hamilton was trailing four-time champion Sebastian Vettel and his Ferrari team-mate, Charles Leclerc, Red Bull's Max Verstappen and his own Mercedes team-mate Valtteri Bottas.
He said he was uncertain that he can reduce Ferrari's advantage in qualifying on Saturday.
"It started out really well so straight away, from the get-go, we were on a good baseline," he said.
"We made a couple of adjustments in the session and we'll do some work tonight to refine it.
"It is quite crucial because we only have two sessions ahead of qualifying to get it right and we have to compact it a little bit more -- and be a bit more precise with the changes we've made.
"I don't know if we have three-tenths to get where the Ferraris are... Obviously, the Ferraris are quick this weekend - or it seems so today. They're generally beating us on the straights, so it'll be interesting to see how that pans out."
Bottas, who will finish as runner-up to Hamilton in the drivers' championship, said he felt optimistic.
"I think we look quite competitive, even for qualifying, but we still have lots more to come."