AUSTIN (AFP) - Torrential rain and electrical storms wiped out afternoon practice at the United States Grand Prix on Friday, plunging Sunday's race into doubt as Texas braced for collateral damage caused by Hurricane Patricia.

The scheduled second practice session was initially postponed indefinitely because marshals at the Circuit of the Americas were unable to work because of the close proximity of the lightning.

Hopes of starting the session later were dashed when it was confirmed that practice had been abandoned because the circuit's medical helicopter would not be allowed to take off for at least two hours due to the conditions.

If the conditions persist, as forecast, the race could be delayed or cancelled, many paddock observers said.

The opening free practice session, in the morning, was also hampered by rain and run in damp conditions.

Germany's Nico Rosberg, who is seeking a victory to keep alive his hopes of wrecking Mercedes team-mate and defending two-time champion Lewis Hamilton's Texan title party, was fastest.

Hamilton was fifth and Sebastian Vettel of Ferrari, who is second in the title race, was sixth, but the session was of little value and gave few clues as to the relative competitive potential of the teams.

The cancellation of Friday's action was not unexpected following forecasts of storms associated with typhoons in the region throughout the weekend.

Hurricane Patricia, which has hit the coast of Mexico, is reported to be the strongest storm ever measured.

Continued bad weather could lead to the cancellation of practice and qualifying on Saturday as well as Sunday.

The last time qualifying took place on a Sunday was at the 2013 Australian Grand Prix when the Saturday session was postponed because of heavy rain.

'NOTHING WE CAN DO'

In 2004, and in 2010, qualifying for the Japanese Grand Prix was abandoned on Saturday because of a typhoon and it took place on Sunday morning.

"There's nothing we can do," said the sport's commercial ring-master Bernie Ecclestone.

"It's alright for those of us who have got jobs and things to do, but I feel sorry for the public.

"They are here to enjoy some racing and all they are getting is soaked in the rain." Rosberg had made a determined first move towards stopping Hamilton's march to his third title when he had topped the times in the morning.

The 30-year-old German, who has to prevent Hamilton beating him by three points in Sunday's race, 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.

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 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.

"There's not a lot you can do in these situations," said Hamilton.

"We've got very little information to work with now, so the rest of the weekend will be intense." 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 laps for Sauber, but 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.

Meanwhile, Briton Jolyon Palmer, son of former Formula One driver Jonathan Palmer, will succeed departing Frenchman Romain Grosjean alongside Venezuelan Pastor Maldonado in the Lotus team next year.

The 2014 GP2 champion has been the team's reserve driver this year, but will be promoted to a race seat next season, the team confirmed in a statement.

Grosjean is leaving Lotus to join American newcomers Haas next season.