Lewis Hamilton set the pace from Mercedes team-mate and title rival Nico Rosberg in first practice at the United States Grand Prix.
Hamilton, who is 17 points ahead of Rosberg with 100 available from the remaining three races of the season, was a comfortable 0.292 seconds clear.
McLaren's Jenson Button was third, split from team-mate Kevin Magnussen by Toro Rosso's Daniil Kvyat.
Ferrari's Fernando Alonso was sixth, ahead of Red Bull's Sebastian Vettel.
Hamilton arrived in Austin for this popular race seeking a fifth consecutive victory after turning a 29-point deficit to Rosberg following the Belgian Grand Prix in August into a sizeable advantage.
But Hamilton said in his BBC Sport column that he was not allowing himself to feel any differently about how he approached the season.
"I can't let my mind drift to something that at the moment is just a dream," Hamilton said. "It has to be business as usual.
"This is just another race, and I want to win it. If I win the next three races and the championship, then that would be amazing, but until then nothing changes."
Hamilton has won nine races to Rosberg's four this year, and admitted on Thursday that it would "suck big time" if he ended up losing the championship only as a result of the controversial introduction of double points at the final race this season.
Hamilton and Rosberg are expected to fight a private battle for victory, with a tight scrap behind them.
Button's performance - just 0.378secs off Hamilton - seemed to reflect an upturn in form in recent races from McLaren, who are hoping to sign Alonso for next season.
Magnussen, on his first experience of the Circuit of the Americas, was 0.668secs adrift of the 2009 world champion.
Vettel, meanwhile, knows he will face a difficult race as a result of Red Bull needing to fit a sixth engine to his car for the remainder of the season, which means a pit-lane start as a penalty.
Red Bull team principal Christian Horner confirmed Vettel would be taking a sixth engine but said Vettel would definitely run in at least some of qualifying on Saturday.
"We'll certainly be taking part in qualifying," Horner said. "But we have to be very careful with our mileage. He has to get through three race weekends with this unit."
Vettel's team-mate Daniel Ricciardo had a troubled session, a problem with the hybrid part of his engine meaning he ran only half the 90 minutes as Red Bull set about changing the part ahead of second practice, which starts at 19:00 GMT.
A low-key session was enlivened by a spin out of Turn 19 by Sauber's Adrian Sutil, and a continuation of Pastor Maldonado's difficult season for Lotus, with a spin at Turn Six.
A number of reserve drivers had an outing, with Felipe Nasr standing in for Valtteri Bottas at Williams and setting the eighth fastest time.
Max Verstappen, who will make his full debut in F1 next year at the age of 17, was 10th fastest, 0.898secs slower than team-mate Daniil Kvyat.
Ferrari spent the session trying out new parts, with Alonso concentrating on a front wing development and team-mate Kimi Raikkonen, who was 12th fastest - 0.9secs behind the Spaniard - on a new rear wing.
After the session, many of the drivers took part in a test of a new system called the 'virtual safety car' aimed at preventing a repeat of the accident at the Japanese Grand Prix that left Jules Bianchi with severe head injuries.
The drivers are given a 'delta' lap time, which they must not better while the virtual safety car is deployed.