Ferrari's Fernando Alonso set the pace in a delayed and truncated first practice session at the United States Grand Prix.
The Spaniard was 0.028 seconds quicker than McLaren's Jenson Button, with Williams's Valtteri Bottas third.
Sauber's Esteban Gutierrez was fourth from the Mercedes of Nico Rosberg and Lewis Hamilton.
The session was subject to an extensive delay because of problems with the weather and the medical helicopter.
Early morning fog meant a 40-minute delay to the start of the session because the medical helicopter could not fly.
Then, when the session did get under way, the cars were out for no more than a few minutes before it had to be stopped because the medical helicopter was not at the track.
Race officials had been told the helicopter was on its way from the airport, but it did not turn up.
It turned out the pilot discovered a radio problem and had to turn around. When it was ready to fly again, air traffic control would not let it cross the airport, and when race director Charlie Whiting was eventually told it was again on its way, he wanted to be sure it was and refused to start the session until he was sure the helicopter was on the ground at the track.
More fog is forecast for Saturday morning, so Whiting plans to ensure the helicopter stays at the track overnight to avoid more problems ahead of final practice.
The session eventually started at 10:30 local time - an hour and a half late and ran for only half an hour.
When the cars started running, Alonso quickly became the pace-setter, a position he held until the end.
The Spaniard started the session not knowing how a back injury suffered in the previous race in Abu Dhabi would respond to being back in the car.
But Alonso, who was wearing tape on his neck and left shoulder to improve circulation, appeared to be able to drive unhindered.
There were impressive performances from many of the new faces in the cars for the session.
Heikki Kovalainen, drafted in to replace Kimi Raikkonen at Lotus for the final two races of the season, was 13th, 0.249secs off his team-mate Romain Grosjean.
Russian Daniil Kvyat, driving for Toro Rosso in first practice for the final two races ahead of his full-time debut with the team next season, was just 0.202secs slower than team-mate Daniel Ricciardo, who is moving to Red Bull.
And American Alexander Rossi, having a run for Caterham in first practice in place of Giedo Van Der Garde, was 0.655secs quicker than team-mate Charles Pic.
The session was not representative of absolute pace - Red Bull's Sebastian Vettel, who tied up the world championship two races ago after dominating the second half of the season, was 18th fastest, with team-mate Mark Webber eighth.