Lewis Hamilton says he had to make "big changes" to his car during the course of free practice for the United States Grand Prix on Friday.
The world championship leader topped both sessions at Austin, and he said he was very happy with his Mercedes after making significant set-up tweaks through the day.
"It's been a really good Friday, really happy with it," Hamilton said. "I always want to start on the right foot.
"We had to make some big changes but I generally made some really good calls with the shift which we ended up making and it worked out really well.
"It's never comfortable, but it was a good day, I was happy in the car. I was making good steps forward in terms of the balance we were changing, so let's hope we can continue it."
Hamilton believes Red Bull looks like a bigger threat than Ferrari so far this weekend, but he accepted that the Italian team had a disrupted day, with Sebastian Vettel spinning off and suffering other problems, leading to the team changing his chassis.
"It's difficult to say, because Sebastian didn't really get much running, but the Red Bulls do look quick," he added.
"But the little bit that we did, looks like we're relatively close to the Red Bulls.
Teammate Valtteri Bottas was less happy with his day after ending FP2 six tenths of a second down on Hamilton, but he said the two Mercedes drivers were trying different things on their cars.
"We got a lot of laps done, a lot of things tried with the set-up, different things with the different cars," said the Finn.
"Today was quite tricky. I wasn't that comfortable, but we did end up with very different set-ups to Lewis. Definitely he had a strong day – me maybe not so much.
"There are a lot of things with the set-up still to be improve, and also from my side many things to be done better, especially with tyre preparation.
"But I'm still confident that if I manage to improve into the right direction it still can be good and be a close battle."