Red Bull's Sebastian Vettel headed McLaren's Lewis Hamilton in first practice at the United States Grand Prix in Austin, Texas.
At F1's first foray into America for five years, the title leader left it late to set a lap nearly a second and a half quicker than Hamilton.
Ferrari's Fernando Alonso, Vettel's rival for the title, was third fastest.
McLaren's Jenson Button ahead of Red Bull's Mark Webber and Ferrari's Felipe Massa filled out the top six.
For much of the session at the new Circuit of the Americas, Hamilton and Alonso traded fastest times as Vettel worked away at learning the track and setting up his car.
But the German came alive in the closing minutes of the session, first beating Hamilton's time by 0.792 seconds and then lowering his mark again to one minute 28.125 seconds - a massive 1.418secs faster than Hamilton.
That caused a sharp intake of breath and underlined exactly why Vettel is a strong favourite for the championship.
The 25-year-old German is 10 points ahead of Alonso heading into the penultimate race of the season, with the finale in Brazil next weekend.
Vettel has won four of the last five races following a major upgrade to the Red Bull car at the end of September, and in Abu Dhabi two weeks ago was third after starting from the pit lane.
He had a slow start to the session, dialling himself into the new 5.5km track, which combines a challenging combination of a fast first sector, a long straight and a twisty final sector.
The low-grip surface caught out several drivers in an incident-packed session.
Force India's Nico Hulkenberg spun twice, once at the uphill Turn One hairpin and later at the slow Turn 13.
The German complained that the rear of his car was "nowhere" but was told by his engineer that it would take time for the track to generate the usual levels of grip.
Several drivers ran wide at the tricky Turn 19, the penultimate corner, including Button and Massa.
And Webber had a spin at Turn Eight, distracted by another car, while Hamilton had a moment when he had to run off the track to avoid the slowing Mercedes of Michael Schumacher.
Several teams were trying out new parts, including Ferrari as they desperately try to close the performance gap to Red Bull to keep Alonso's dwindling hopes alive.
The Spaniard played around with front wings, under-nose turning vanes and a new diffuser.