Austin - Lewis Hamilton topped the times for Mercedes in Friday's rain-abbreviated second practice for this weekend's United States Grand Prix and was then buoyed further when title rival Sebastian Vettel was hit with a three-place grid penalty for Sunday's race.
In a session that only burst into life for the final 40 minutes as the track dried, a relaxed Hamilton clocked a best lap in one minute and 48.716 seconds that lifted him clear of the pack.
It confirmed a "double-top" for the Briton who had also been fastest in the morning's opening practice session and who needs to outscore Vettel of Ferrari by eight points to wrap up his fifth drivers championship.
The German struggled through a difficult day of slithering in the rain and then picked up a three-place grid penalty for Sunday's race for speeding under red flags during the morning session.
He was 10th in the second session, but that meant little as only a handful of drivers put in a serious effort to clock competitive laps in the conditions, partly due to a shortage of suitable full wet and intermediate tyres available at the Circuit of the Americas.
Vettel and Ferrari had hoped the stewards, who spent several hours investigating his speeding offence in the morning, might take a more relaxed and "common-sense" approach to his misdemeanor.
Instead, his hopes of stopping Hamilton from claiming a fifth world title this weekend suffered a setback.
The Ferrari driver was called to see the stewards after being reported for a marginal speeding offence committed under "red flag conditions".
His penalty followed two similar examples.
Esteban Ocon of Force India and Daniel Ricciardo of Red Bull were given three-place penalties for speeding under red flags at the Japanese and Australian Grands Prix respectively.
Vettel was reported for speeding after Charles Leclerc of Sauber brought gravel back onto the circuit after a spin.
All drivers are expected to slow down and drive back to the pits when red flags are waved.
In the rainswept session, Hamilton was top ahead of Pierre Gasly of Toro Rosso, Max Verstappen of Red Bull, Fernando Alonso of McLaren and Nico Hulkenberg of Renault.