Sebastian Vettel once again topped the timesheets at the end of day one of a grand prix meeting, but warned his rivals that there was more to come from Red Bull at the Circuit of the Americas.

The German had languished down in 18th position on the morning timesheets - behind even Toro Rosso newcomer Daniil Kvyat - as Red Bull worked through its development programme for the still relatively new Austin venue, but came good after lunch, posting a 1min 37.305secs effort to move into top spot on the medium compound Pirelli tyres.

Vettel completed a combined 35 laps in the later session, running both the hard and medium compounds, but appeared to be baulked on what could have been better laps before eventually setting the pace on used rubber.

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"The circuit was quite slippery today," he revealed, "I was happy with the car, but you always know you can improve here and there.

"I think we got through the programme, [and] we were able to test some things, some were good and some not so good, but we will see. Ferrari looked quick this morning and McLaren could be a surprise here, while Lotus and Mercedes will be strong as normal. The track will improve now unless it rains, the car worked well on the tyres, so we'll see what we can do on Sunday."

RBR team-mate Mark Webber had held top spot for some time in the afternoon session, and also looked strong on the harder Pirellis, but was unable to respond to Vettel's improvement, leaving him second overall on the day.

"Today was all about tyres again; getting info on them and working out how they work best," he reported, "It's nice to drive the car on fresh tyres and it was good to be out there on the circuit on what was a clear day in the end. We were working on both long and short runs [and] the car doesn't feel mega yet, there's still some work to do, but we'll work on that overnight."