Lewis Hamilton - 2nd/4th: "What a great turn-out for a Friday! I never expected to see so many people in the grandstands - it makes me feel even more excited for Saturday and Sunday.
"This is a great drivers' track - it's very tough to pull together three perfect sectors. Personally, I find the final two sectors easier than the first; those high-speed changes of direction need a good set-up, and balancing that requirement against the demands of the slow-speed stuff is tricky.
"Turn One probably looks more exciting from the outside than it does from inside the cockpit - it's certainly not easy, though. You can get oversteer, or lock up, and it's tricky to get a perfect line through there.
"Overall, we've had a good day. Our main difficulty this morning was switching on the tyres in the cooler conditions. But we'll make some changes overnight - there are definitely some improvements we can make."
Jenson Button - 4th/5th: "The circuit was very slippery this morning - as all new track surfaces are, particularly as it was also very cold. But this afternoon's warmer temperatures meant we could get a better feel for the circuit in FP2.
"Turn One to Turn Nine is a brilliant and free-flowing section - particularly if the car is working well. Turn One is a strange corner though - with a very wide entry rather like Turn Three in India. Turn Three here - the start of the esses - is fantastic though. It's quicker than Becketts at Silverstone. It's very unsuual to find a section of corners like this on a modern Formula 1 track. I love it.
"Nevertheless, I don't think overtaking is going to be easy here, because it'll be so tricky to stay close to the car in front of you. But the circuit is great fun to drive, and I hope we can put on a great race."
Martin Whitmarsh: "It's easy to turn up at a new grand prix facility and almost take it for granted; I don't think anybody in the F1 paddock did that here. Make no mistake: this is a world-class facility - built from nothing - and I'm sure it will quickly earn a reputation for being one of the greatest racetracks in the world.
"The scale of achievement - on both a human level and an engineering level - is truly astonishing. The high-speed esses in the first sector are a real test for both man and machine, whilst the excellent facilities - for teams and spectators alike - make it feel like a very 'human' place.
"Most of all, the people who have visited here in their thousands this weekend deserve our recognition and respect. We've been made to feel very welcome in this beautiful city ever since we started arriving earlier in the week, and it's very heartening to see a community so strongly supporting the success of their event.
"Overall, the return of Formula 1 to the United States is a massively positive step - and, after an encouraging first day of practice, let's hope we can put on a show that will make everyone proud."