After sitting near the bottom of the timing sheets in a foggy and chaotic opening practice session, the 26-year-old German sizzled under the blazing afternoon Texas sun and lapped the Circuit of the Americas with a best time of one minute 37.305 seconds.
Vettel, who won his fourth successive title in India last month and now has 11 wins this year, was 0.115 seconds faster than Australian team mate Mark Webber who cemented the Red Bull one-two.
No driver has ever won eight races in a row in a single season, but Vettel equalled Michael Schumacher's seven earlier this month in Abu Dhabi and is favourite to win again this weekend.
The two Red Bulls were followed by the two Mercedes drivers with Germany's Nico Rosberg third and Britain's Lewis Hamilton, winner of the inaugural race in Austin last year with McLaren, ready for another strong weekend in fourth.
Heikki Kovalainen, the Finn who is replacing compatriot Kimi Raikkonen at Lotus for the last two races after the 2007 champion pulled out to have back surgery, was an impressive fifth fastest in the afternoon.
The Finn's French team mate Romain Grosjean was eighth in the same session.
Lotus are fighting with Ferrari and second-placed Mercedes for the coveted, and lucrative, runner-up slot in the constructors' championship already won by Red Bull.
McLaren had another tough day with Jenson Button handed a three place grid penalty for Sunday's race after stewards ruled the Briton had overtaken a car while the morning session was under a red flag.
The Briton was ninth quickest in the afternoon ahead of Ferrari's Fernando Alonso, who was given the all-clear to race after a medical on Thursday.
The Spaniard, who hurt his back when he hit a kerb at high speed during the race in Abu Dhabi, had topped the delayed and truncated morning practice session but could not match that pace in the afternoon.
The day got off to a stuttering start when teams and drivers were kept waiting for 40 minutes as heavy fog shrouded the circuit carved out of the south Texas scrubland.
Formula One's governing body, the FIA, said the delay was necessary because medical helicopters would not be able to take off or land if needed.
When it did eventually start, the session was soon red flagged again with officials waiting for an absent helicopter to land.
(Editing by Alan Baldwin)
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