US F1 Grand Prix: Mercedes' Nico Rosberg tops damp first practice

As had been the case in the last two events at Suzuka and Sochi, wet conditions dominated early proceedings as a heavy downpour soaked the Circuit of the Americas just over an hour before the session began.

Fortunately the rain relented with 45 minutes to the start, and although it stayed away for the 90 minutes of on-track action that followed, it ensured the circuit was at best damp throughout.

After the initial installation lap was conducted on Pirelli's full wet tyres, the field switched to the intermediates to carry out more meaningful running thereafter.

Unlike many opening practice sessions when the cars sit in the garage on the basis of drier weather being forecast as the event progresses, on this occasion with rain set for most of the weekend, the teams opted to churn out the laps.

Come the conclusion it was Rosberg that was quickest, with the Mercedes driver setting a time of 1m53.989s, finishing 1.235 seconds ahead of Red Bull's Daniil Kvyat, with the latter's team-mate Daniel Ricciardo a further four tenths adrift.

Although Renault has an updated power unit available for Red Bull to use from this weekend, for first practice the team continued to use its old-spec engine, which it is expected to retain for the remainder of the weekend.

The system is also available to Toro Rosso, however, Carlos Sainz Jr confirmed on Thursday the team will not run with it at all for the remainder of the campaign.

It was the Spaniard who finished fourth on the timesheet, 1.678s down, and just 0.026s ahead of Mercedes' Lewis Hamilton, who can clinch his third world title this weekend.

Ferrari's Sebastian Vettel and team-mate Kimi Raikkonen were sixth and eighth, the duo separated by six tenths of a second and either side of the second Toro Rosso of Max Verstappen.

Ferrari is running an updated power unit this weekend that will result in 10-place grid penalties for both drivers as it is the fifth system the Scuderia is using this year.

McLaren's Fernando Alonso was ninth, 2.850s back, with the double world champion using Honda's 'phase four' power unit that will form the bedrock of its 2016 system.

Alonso, who has so far expressed positivity about the new engine, finished seven tenths of a second ahead of team-mate Jenson Button, still running the older unit as he will not be handed the latest version until next weekend's race in Mexico.

Button finished 13th, with Williams's Valtteri Bottas and Felipe Massa, along with Force India's Sergio Perez all ahead of him in 10th, 11th and 12th respectively.

Manor duo Alexander Rossi, on home soil, and Will Stevens naturally brought up the rear in terms of times set in 18th and 19th, the latter almost eight seconds back.

Due to a gearbox problem Lotus's Pastor Maldonado failed to set a time, only managing a single installation lap.

PRACTICE ONE TIMES: