US Grand Prix: Nico Rosberg Strikes First Texas Blow Against Lewis Hamilton

Nico Rosberg made a determined first move towards wrecking Mercedes team-mate Lewis Hamilton's Texan title party when he topped the times in opening free practice for this weekend's United States Grand Prix.

The 30-year-old German, who has to prevent Hamilton beating him by three points in Sunday's race at the Circuit of the Americas to keep his own challenge alive, clocked a best lap of one minute and 53.989 seconds on a damp, drying track.

Hamilton, feeling his way cautiously, was one of the majority of drivers who were slow to join the fray and wound up fifth quickest after a relatively desultory session in poor conditions.

Heavy rain had soaked the circuit ahead of the action and has been forecast for the rest of the weekend, a factor that may play havoc with the outcome as defending double world champion Hamilton bids to wrap up his third title three races remaining.

Rosberg's best time left him more than 1.2 seconds clear of the Red Bull pairing of Daniil Kvyat and Daniel Ricciardo with Carlos Sainz fourth for Toro Rosso.

Four-time champion Sebastian Vettel, who is second behind Hamilton in the title race, was sixth for Ferrari after clocking only 10 laps in the session.

Vettel has already been handicapped ahead of Sunday's race by being hit with a 10-place grid penalty after Ferrari introduced new engines for the event.

Rosberg completed seven laps in the 90 minutes while Hamilton did only four.

The busiest men were the two McLaren Honda drivers, Jenson Button, the 2009 champion, and two-time champion Fernando Alonso, who has an upgraded engine in his car this weekend.

Button completed 20 laps and Alonso 18 while Raffaele Marciello did 19 for Sauber.

However, it was mostly meaningless stuff in difficult conditions as the drivers acquainted themselves with the circuit on full wet tyres before switching briefly to intermediates as the track dried out.