Lewis Hamilton's hopes of clinching his fifth Formula One world championship at the U.S. Grand Prix were boosted when Ferrari rival Sebastian Vettel was handed a three-place grid penalty for a practice infringement.
Hamilton had already dominated both rainswept practice sessions when the German's penalty was confirmed by race stewards.
Vettel, whose season has been punctuated by mistakes, added another error to the list when he was found to have gone too fast when red warning flags were waved during the morning running.
"I saw the red flag, I slowed down, they found it took too long and I think it's pretty straightforward," the unhappy German told Sky Sports television.
"But the rules are clear as well, it's the first time we had this in the wet and you literally have to stop to 30 or 40kph, which I should do next time," he added.
"I'm not sure it's the right thing to do if there's someone right behind you, but it's more important you don't get the penalty."
Hamilton has won six of the past seven races and is chasing his fifth successive victory on Sunday.
If he wins, then Vettel must finish second to deny the Briton a title that would elevate him alongside the late great Argentine Juan Manuel Fangio as only the third driver to win five or more titles.
Hamilton has a 67-point lead and will be champion for the fourth time in five years if he scores eight points more than Vettel, who cannot now start higher than fourth.
The Mercedes driver put down an immediate marker in Friday's opening session by lapping 1.304 seconds quicker than the next man, his own Finnish team mate Valtteri Bottas.
He was more than a second faster than anyone else in the second session as well, with Toro Rosso's Pierre Gasly making the most of the treacherous conditions to take second place on the timesheets.
Vettel meanwhile was fifth and 10th respectively while Red Bull's Dutch 21-year-old Max Verstappen, always a threat, was third in both.
Ferrari's Kimi Raikkonen was sixth and ninth.
Hamilton won his third championship at the Texan track in 2015 and has been almost unbeatable in Austin, winning five of the six races there since Texas first appeared on the calendar in 2012.
With the rain coming down steadily overnight, but easing slightly before the day's action started, Friday's times were comparatively slow and Hamilton took his time before venturing out on track.
The red flags came out in the first session after Sauber's Charles Leclerc, Vettel's future Ferrari team mate, spun at turn eight and went off, scattering gravel across the track.
There was little track action for most of the afternoon due to the conditions, which lifted in time for 15 of the 20 drivers to set a time.
New Zealand Toro Rosso driver Brendon Hartley only took part in the second practice session and was sixth fastest.