Lewis Hamilton believes Mercedes "lost some ground" after dominating the opening practice session for the United States Grand Prix which has left it with work to do.
Team-mate Valtteri Bottas spearheaded a Mercedes one-two at the Circuit of the Americas in FP1, with Red Bull's Max Verstappen the best of the rest but almost a second adrift.
In FP2, Sergio Perez led the way after what would have been the best lap of the day from Hamilton was deleted as he exceeded track limits at the penultimate corner.
"Generally, it's been a good day but [there is] still work to do, unfortunately," reviewed seven-time champion Hamilton on the day's proceedings.
"FP1 was a really good session, [but then we] made some changes, and I wasn't happy in P2.
"Still not such bad pace but I think we lost some ground to everyone, so we have some work to do as always."
Unusually high temperatures at this time of the year in Austin, hitting 30 degrees centigrade and raising the track condition to almost 50 degrees, appeared to play a key role in the soft tyres, in particular, degrading more quickly than anticipated.
Hamilton has suggested, though, a recent resurfacing of the circuit to smooth out bumps was likely the main cause.
"I don't think it's the heat necessarily, I think it's literally just the track surface is rougher than before," assessed Hamilton.
"It's a struggle to keep the temperature out of the tyres. It's impossible pretty much so there's more deg than we've experienced before."
As a consequence, that has affected car setup, with Hamilton adding: "Definitely it's difficult because you are trying to get a quali setup and also a setup that will work on the long runs.
"It's not an easy car to set up in general but there were plenty of findings we should be able to navigate well."