Sebastian Vettel and Mark Webber assumed familiar positions at the head of the timesheets at the end of Friday practice at the US Grand Prix, confirming that the world champions will again be the team to beat in Texas.

Both drivers took turns in P1 before Vettel settled the issue with a 1min 37.305secs effort on the medium Pirellis. Before that, Webber had set the pace, and looked strong on the harder compound, but could not respond to Vettel's late-session time and had to settle for second spot. More worryingly, however, Vettel appeared destined for better times earlier in his run, but found traffic on both occasions, so has the potential to improve further on day two.

Behind Webber, Mercedes marked itself out as the team most likely to challenge Red Bull, with Nico Rosberg and Lewis Hamilton filling third and fourth spots on the timesheet. The Briton, however, again admitted that he had not been at his best, with a series of early errors negating the effect of running a new chassis on the faster tyres. Hamilton ended the day a couple of tenths off his team-mate, and more than half a second off Vettel.

Heikki Kovalainen rounded out the top five, justifying Lotus' decision to hire him as Kimi Raikkonen's replacement rather than opting for official reserve Davide Valsecchi. The Italian admitted before the session that Enstone had opted for experience in the battle for third place in the constructors' table, and Kovalainen put all of his to good use as he out-paced team-mate Romain Grosjean in a car still unfamiliar to him. The Finn was seen complaining about the support he was receiving from his seat, but continued to run with the original cast as the team vowed to address the problem overnight.

Grosjean, meanwhile, was only eighth fastest, as both Saubers squeezed in ahead of him. Esteban Gutierrez claimed sixth overall, two tenths off Kovalainen ahead of him, with Hulkenberg closing to within a matter of hundredths of the Mexican on his best lap. Grosjean then lapped a single thousandth off the German, who may yet become his team-mate in 2014.

Jenson Button, having absorbed the news that he will be subject to a three-place grid penalty on Sunday, claimed ninth place, edging out Ferrari's FP1 pacesetter Fernando Alonso, the Spaniard being the first driver over a second off Vettel, and driving a Ferrari that did not appear comfortable as the Circuit of the Americas warmed up.

Unlike FP1, there was no delay or interruption to the session, which ran to its full 90-minute duration as a result. Adrian Sutil, Felipe Massa and Sergio Perez were next up behind Alonso, each lapping in the 1min 38s, with Toro Rosso's Daniel Ricciardo the first man in the 1min 39s. Team-mate Jean-Eric Vergne returned after seeing 2014 partner Daniil Kvyat get his chance to drive in FP1, but the Frenchman had to make do with 17th, his 1min 39.579secs effort three-tenths off Ricciardo.

Countryman Jules Bianchi also returned to action after seeing his car parked at the side of the road by Rodolfo Gonzalez in FP1, but neither Marussia appeared happy and languished a long way off the pace of even their closest rivals. Max Chilton, who ended the afternoon session with a spin entering turn 20, was 8.9secs adrift of Vettel, and nearly six seconds slower than the worst of the Caterhams, while Bianchi was a further 1.5secs behind.