Vettel topped both practice sessions ahead of the United States Grand Prix as the Circuit of The Americas threw open its doors for business to herald the return of Formula One to America after a five-year absence.
The 25-year-old German finished an astonishing 1.4 seconds clear of the opposition after the opening 90 minutes around the undulating 5.513km track - the largest margin at the end of any practice outing this season.
Vettel, who needs to finish 15 points clear of Alonso on Sunday to become only the third man in F1 history to win three consecutive titles – Juan Manuel Fangio and Michael Schumacher the others – finished with a lap time of one minute 37.718secs.
And in the land of the Texas longhorn, it was the Red Bulls who had the edge as team-mate Mark Webber was second quickest with a time of 1:38.475 – 0.757secs adrift.
As for Alonso, the Spaniard in his Ferrari had to settle for third, just eight thousandths of a second adrift of Webber, but like the Australian almost eight tenths of a second down on Vettel.
McLaren duo Lewis Hamilton and Jenson Button were fourth and fifth quickest, both a full second behind, followed by the second Ferrari of Felipe Massa.
Mercedes pair Nico Rosberg and Michael Schumacher were seventh and 10th, sandwiching Williams’ Bruno Senna and Sauber’s Kamui Kobayashi, Vettel having a 2.3secs gap over Schumacher.
The Lotus’ of Kimi Raikkonen, winner last time out in Abu Dhabi, and Romain Grosjean appear to be struggling as they were 11th and 13th