Kimi Raikkonen admitted that it was difficult to find the limits of the Circuit of the Americas track due to the lack of grip on day one in Austin.
A number of drivers suffered spins or minor trips off track in the two Friday practice sessions as they attempted to learn the new circuit, while Raikkonen was also charged with re-evaluating exhaust updates on the Lotus E20.
The Finn ended the day with the eleventh quickest time and he said an interesting day lies ahead on Saturday as the circuit continues to evolve.
“It's a nice circuit to drive; the sectors are quite different so there's a good challenge there,” he said. “Of course, like with any new track there was no grip at the start of the day which doesn't give you a very good feeling, but as the grip improves it will be more fun to drive for sure.
“This morning was very slippery, this afternoon less so but you still can't really push and it's a bit tricky to find the limit. There's a lot of improvement to come - from the track surface and from the setup of the car - so qualifying will be pretty interesting.”
Team-mate Romain Grosjean was 13th quickest in the second car and he predicted that the lack of grip would play a crucial role in what happens through the remainder of the weekend.
“The most noticeable thing about the circuit at this stage is the lack of grip; it's really, really slippery out there,” he said. “The layout itself looks very nice but we're a long way off being able to push the car anywhere near the limit, particularly through Turns 7-9 and towards the end at Turn 19 where it's tricky to predict how the car will behave.
“The good thing is that the car felt comfortable on long runs with the hard tyre so we have a good starting point, but there is still a little bit of work to do with the mediums. We still have tomorrow morning and I think the track will continue to improve right until the end of the weekend, so qualifying will be interesting for sure.”