Hamilton was two tenths off Sebastian Vettel's headline time by the end of Friday at Interlagos, although the German's lap was compromised by Valtteri Bottas.
The Finn also got in Hamilton's way as qualifying sims ramped up in Brazil.
And while Hamilton says there remains plenty of time on the track for the Silver Arrows, he expects the Scuderia will also make gains, although Ferrari's efforts will be hampered across the weekend by Charles Leclerc's impending 10-place grid penalty.
Hamilton said: "It started out really well so straightaway from the get go we started on a good baseline and we just made a couple adjustments in that short space of time we had in the session and then 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 today. They're generally beating us on the straights so it'll be interesting to see how that pans out."
GPFans is a multi-platform, multi-language brand dedicated to Formula One coverage. We bring you all the ins and outs of the sport, 24/7, everything from up-to-the-minute news and features to the latest viral stories and clips.
We believe that a new generation of exciting, outspoken drivers will make F1 more popular than ever before, and we want to give our users access to as much of their heroes as possible, on and off the track. From Lewis Hamilton to Max Verstappen, Daniel Ricciardo to Sebastian Vettel, we provide in-depth analysis of every every Grand Prix in the season, from Australia to Abu Dhabi.
With Formula One under the new ownership of Liberty Media, how the sport is being covered is evolving, and GPFans will look to be at the heart of this progression into new media, as one of the fastest-growing sites covering the king of motorsports.