Sebastian Vettel did little to debunk the theory that he will run away with this weekend's United States Grand Prix by topping the practice timesheets for a third time in as many sessions at the Circuit of the Americas.
As he had in both of Friday's sessions, the German left it late before punching in a time that left his rivals reeling, although the initial one-second margin created by his 1min 36.490secs effort towards the end of Saturday's hour-long outing was notably the result of bolting on the softer medium 'option' tyre and posting a lap before most of the others had had chance. When they responded the advantage was trimmed, although Vettel still had nearly three-tenths in hand over second place once the dust had settled.
Lewis Hamilton was once again the closest thing to opposition for Vettel, the Briton continuing to show that he had a handle on the Austin facility by running at the head of the times for much of the session. Although McLaren team-mate Jenson Button dared to occupy top spot early on, once Hamilton had asked what the gap between them was, it disappeared almost immediately, and it was until Vettel's late assault that the Briton was finally deposed.
Pastor Maldonado and Fernando Alonso both mounted their own late moves up the order, the Venezuelan briefly usurping Hamilton from P2 before the Briton completed his run on the option Pirellis. He would go on to hold down third place, and only narrowly miss joining Vettel and Hamilton in the 1min 36s, while Alonso took fourth for Ferrari just eight-hundredths further back.
Nico Rosberg rounded out the top five for Mercedes ahead of the second Ferrari, Felipe Massa filling sixth spot for the third straight session after escaping having his nose chopped by the other Mercedes of Michael Schumacher early on. The Scuderia, however, suffered for deciding to complete its pit-stop practice in the opening part of the session, at the same time scrubbing tyres ahead of qualifying. Mark Webber had been ahead of both drivers, having posted the fifth-best time on his final run, but eventually settled for seventh, just ahead of Sergio Perez, Nico Hulkenberg and Button, who was also subsumed in the final phase.
Perez was lucky to have been able to continue his session after a clash with Charles Pic left both navigating their way out of the run-off areas. Initial blame was laid at the Mexican's feet for attempting a move through a gap that was already closing, but later analysis suggested that Pic had been travelling a lot slower than someone who should have been taking the racing line. Neither car was badly damaged in the resulting collision, although Pic could only manage P22 on his return.
Behind Button, Bruno Senna enjoyed another encouraging session, edging Schumacher for eleventh, while Abu Dhabi winner Kimi Raikkonen again missed the top ten despite being the best of the Lotus drivers. The Finn was 1.2secs off the pace in 13th spot, but still fared better than team-mate Romain Grosjean, whose session ended early with suspected gearbox failure that could lead to a potential grid penalty.
Kamui Kobayashi, Daniel Ricciardo and Paul di Resta wound up splitting the two Lotus men, with Jean-Eric Vergne next up after his session was also cut short. The Toro Rosso driver pulled off with his right-front wheel flapping midway through the session, the result of an energetic meeting with one of CotA's kerbs.
Timo Glock won the third division battle for Marussia, ahead of Vitaly Petrov, Heikki Kovalainen and Pic, with the two HRTs more than two seconds adrift of the French rookie.
Grip continued to be an issue on the all-new surface, despite the best efforts of jet driers overnight, but lap times continued to come down, with Vettel's benchmark 1.3secs faster than he had managed on the opening day.