Austin, Texas - Lewis Hamilton kept his powder dry in a wet and largely meaningless first practice on Friday (Oct 23) for a US Grand Prix that could seal the Briton's third F1 world championship.
The Mercedes driver completed just four timed laps and ended up fifth overall, with team mate Nico Rosberg leading the timesheets.
A second faster
Rosberg lapped more than a second faster than anyone else, the German's best time of 1min53.989 seconds comparing to the next quickest of 1min55.224 set by Daniil Kvyat for Red Bull.
Red Bull's Daniel Ricciardo was third and Toro Rosso's Carlos Sainz fourth on a rain-soaked track with more wet weather expected later for the second session and much heavier thunderstorms over the weekend.
Parts of Texas are on flood alert with talk of possible extreme rainfall in the Austin area on Saturday when qualifying takes place.
Further south, Mexico, which hosts the next round of the championship next weekend, is braced for the arrival of one of the most powerful storms in history with Hurricane Patricia heading towards the Pacific Coast.
Hamilton needs to beat Ferrari's Sebastian Vettel, who has a 10-place grid penalty for Sunday's race due to an engine change, by nine points and Rosberg by two to retain his title and become a triple champion.
A Mercedes 1-2, with Hamilton first, would do the trick and the Briton has a great record in Austin with two wins in the last three races there.
Friday's session started wet, although conditions were changeable, but there were few incidents despite a couple of spins.
Finland's Valtteri Bottas gave his pit crew a scare, forcing some mechanics to jump out of the way when he came in and skidded to a halt.
"Sorry for that," he said over the radio as he departed again.
Vettel was sixth fastest, with Toro Rosso's Max Verstappen seventh and Ferrari's Kimi Raikkonen eighth.
Manor Marussia's Alexander Rossi, who will be the first American to compete in a U.S. Grand Prix since 2007, was 18th of the 20 drivers.
2015 United States Grand Prix 1st Practice Results
1. Nico Rosberg (Germany) Mercedes 1min53.989
2. Daniil Kvyat (Russia) RedBull - Renault 1min55.224
3. Daniel Ricciardo (Australia) RedBull - Renault 1min55.592
4. Carlos Sainz Jr (Spain) Toro Rosso - Renault 1min55.667
5. Lewis Hamilton (Britain) Mercedes 1min55.693
6. Sebastian Vettel (Germany) Ferrari 1min55.710
7. Max Verstappen (Netherlands) Toro Rosso - Renault 1min55.969
8. Kimi Raikkonen (Finland) Ferrari 1min56.326
9. Fernando Alonso (Spain) McLaren 1min56.839
10. Valtteri Bottas (Finland) Williams-Mercedes 1min56.866
11. Felipe Massa (Brazil) Williams-Mercedes 1min57.081
12. Sergio Perez (Mexico) Force India - Mercedes 1min57.139
13. Jenson Button (Britain) McLaren 1min57.495
14. Nico Hulkenberg (Germany) Force India - Mercedes 1min57.518
15. Romain Grosjean (France) Lotus - Mercedes 1min58.319
16. Raffaele Marciello (Italy) Sauber - Ferrari 1min59.431
17. Marcus Ericsson (Sweden) Sauber - Ferrari 1min59.743
18. Alexander Rossi (US) Marussia - Ferrari 2min01.154
19. Will Stevens (Britain) Marussia - Ferrari 2min01.907
20. Pastor Maldonado (Venezuela) Lotus - Mercedes