Sebastian Vettel completed a clean sweep of Friday's two free practice sessions at the Circuit of the Americas despite being forced to miss part of the afternoon outing with a water leak.

The German, who had blitzed the opening 90-minute session with a late lap on nearly new tyres to lead the field by 1.4secs, only enjoyed half that margin over his nearest pursuer after lunch but, given that second place went to Red Bull team-mate Mark Webber, it did not provide much encouragement to the rest of the pack.

This time, Vettel didn't wait as long to hit top spot, but had to sit out nearly 20 minutes of the session after his RB8 developed a leak from its left-hand sidepod, much as Webber's had in FP1. Refusing to simply kick his heels at the back of the garage, however, Vettel pored over the Australian's data during his enforced break, and then returned with a vengeance to vault almost immediately into a lead he would not lose.

Although he did not subsequently set the pace during heavy fuel race simulation runs - that honour went, by most stopwatches, to McLaren's Lewis Hamilton - Vettel's performance is ominous for the championship battle, which most hope will be prolonged until next weekend's Brazilian finale. The Red Bull proved faster than Fernando Alonso's Ferrari through the final portion of the session, suggesting that, barring misfortune, Vettel should at least leave Texas with his task made easier at Interlagos.

Alonso ended the day third fastest behind the two Red Bulls, and will be encouraged by at least lapping in the same tenth as Webber, even if Vettel enjoyed a comfortable margin over both. Alonso was clearly frustrated by traffic on a couple of occasions, and pitted late in the session after making an error and running wide at the penultimate corner.

The Spaniard was in the middle of a closely-matched group scrapping over second spot, for it wasn't just Webber keeping him company, but both McLarens too. Hamilton, who had duelled for top spot with Alonso in FP1 before pulling out a gap and then being blasted by Vettel's late flier, slotted into fourth overall, just a couple of tenths further back, while team-mate Jenson Button also lapped in the 1min 38.7s to round out the top five.

Felipe Massa echoed his first session time to uphold sixth spot in the second Ferrari, with Nico Rosberg and Michael Schumacher hauling their Mercedes into the top ten, sandwiching the recovering Bruno Senna - who transformed 22nd in FP1 into eighth this time around - and Kamui Kobayashi at the foot of the leading group.

Kimi Raikkonen and Romain Grosjean appeared to have secured eleventh and twelfth for Lotus until Pastor Maldonado found pace late on to split the black-and-gold machines. The Venezuelan, however, ended his session surrounded by Williams personnel brandishing fire extinguishers after an engine failure in the final two minutes threatened to set his car alight.

Sergio Perez was 14th overall in the slower of the two Saubers, and was only just ahead of the two Toro Rossos, which appear to have found some speed as Daniel Ricciardo and Jean-Eric Vergne slotted themselves in above both Force Indias.

The Silverstone-based team at least avoid the ignominy of being matched by the usual tail-enders, where Heikki Kovalainen led the way for Caterham, but lapped a full second off the back of the midfield. The Finn was also among those that continued to push the limits of adhesion on the still slippery Austin circuit, although he was in exalted company as the likes of Vettel, Hamilton and Massa all also explored regions beyond the prescribed racing line.

HRT, as expected, brought up the rear, its parlous financial state restricting the running available to Pedro de la Rosa and Narain Karthikeyan as it continued to 'life' components that it had little or no replacement for. Sitting out was particularly harsh on the Indian as he had had to give over his car to Ma Qing Hua in the morning session, but he did well to lap within seven-tenths of his Spanish team-mate.