Mercedes driver Nico Rosberg set the pace in a rain-affected first practice at the United States Grand Prix.
The German, needing to beat team-mate Lewis Hamilton to retain his slim title hopes, headed Red Bull’s Daniil Kvyat.
The session marked the start of what promises to be a difficult weekend, with Austin, Texas, expected to be affected by heavy storms and rainfall.
This is due to Hurricane Patricia, the strongest tropical storm ever recorded, which is heading towards Mexico.
Overnight and morning rain stopped before the first session but is expected to return ahead of the second session at 14:00 local time (20:00 BST) before worsening overnight.
Weather forecasts predict extreme levels of rain on Saturday, perhaps as much as eight to 12 inches falling in one day in certain places.
That raises the very real possibility that qualifying could be delayed until Sunday morning.
The conditions on Friday morning here might well be the best until Sunday, but running was limited as teams tried to restrict their use of the wet and intermediate tyres.
They have only three sets of the heavily treaded ‘extreme wets’ and four of the shallower-treaded ‘intermediates’ for the entire weekend and these will need to be in their best condition in qualifying and the race.
Rosberg, who set his time relatively early in the session, was 1.235 seconds quicker than Kvyat.
The Russian’s team-mate Daniel Ricciardo was third fastest, just under 0.4secs behind his fellow Red Bull driver and only 0.075secs quicker than Toro Rosso’s Carlos Sainz.
Hamilton was fifth, 1.704secs slower than Rosberg and 0.017secs quicker than Ferrari’s Sebastian Vettel.
The Briton needs to beat Vettel by nine points and Rosberg by two to clinch the title this weekend.
A victory with Rosberg second and Vettel third would be enough for the Englishman to clinch his third title.
His task was eased when it emerged that both Ferrari drivers will have 10-place grid penalties for using a fifth engine this weekend.
