Austin - Championship leader Lewis Hamilton topped the times for Mercedes by half a second ahead of title rival Sebastian Vettel of Ferrari in Friday morning's opening free practice for this weekend's United States Grand Prix.
On a damp and overcast day at the Circuit of the Americas, the 32-year-old Briton clocked a best lap in one minute and 36.335 seconds to outpace the four-time champion German by 0.593 seconds.
Hamilton leads Vettel by 59 points with four races remaining and will seal his fourth drivers' title in Sunday's race if he can win and Vettel fails to finish in the top five.
Hamilton's team-mate Valtteri Bottas was third fastest ahead of Max Verstappen of Red Bull, Stoffel Vandoorne for McLaren Honda and Felipe Massa of Williams.
Verstappen, who on Friday signed a new three-year contract keeping him at Red Bull until 2020, topped the times during the session, but could not match the leading cars on super-soft tyres in the closing minutes.
Vettel's Ferrari team-mate and 2007 champion Kimi Raikkonen was seventh ahead of Esteban Ocon and his Force India team-mate Sergio Perez with Carlos Sainz 10th in his first run for Renault after leaving Toro Rosso.
Debutant Brendon Hartley, who became the first New Zealander to take part in a Grand Prix for 33 years, was 14th fastest but will start Sunday's race from the back of the grid after his Toro Rosso team changed his power unit.
United States Grand Prix practice times from first session on Friday:
Hamilton (GBR/Mercedes) 1:36.335 (18 laps), Sebastian Vettel
(GER/Ferrari) 1:36.928 (20), Valtteri Bottas (FIN/Mercedes) 1:36.979
(10), Max Verstappen (NED/Red Bull-TAG Heuer) 1:37.339 (21), Stoffel
Vandoorne (BEL/McLaren-Honda) 1:37.352 (26), Felipe Massa
(BRA/Williams-Mercedes) 1:37.570 (23), Kimi Rikknen (FIN/Ferrari)
1:37.598 (20), Esteban Ocon (FRA/Force India-Mercedes) 1:37.808 (25),
Sergio Prez (MEX/Force India-Mercedes) 1:37.861 (20), Carlos Sainz Jr
(ESP/Renault) 1:38.093 (24), Kevin Magnussen (DEN/Haas-Ferrari) 1:38.408
(16), Lance Stroll (CAN/Williams-Mercedes) 1:38.534 (25), Nico
Hlkenberg (GER/Renault) 1:38.904 (19), Brendon Hartley (NZL/Toro
Rosso-Renault) 1:39.267 (28), Romain Grosjean (FRA/Haas-Ferrari)
1:39.336 (17), Daniel Ricciardo (AUS/Red Bull-TAG Heuer) 1:39.366 (14),
Sean Gelael (INA/Toro Rosso-Renault) 1:40.406 (25), Marcus Ericsson
(SWE/Sauber-Ferrari) 1:40.448 (19), Charles Leclerc (MON/Sauber-Ferrari)
No time set: Fernando Alonso (ESP/McLaren-Honda) (4)