* Hamilton quickest in both sessions
* Mercedes drivers separated by just 0.003 of a second (Adds second session)
By Alan Baldwin
AUSTIN, Texas, Oct 31 (Reuters) - Formula One championship leader Lewis Hamilton and his title rival team mate Nico Rosberg were separated by mere fumes on Friday in a familiar Mercedes one-two in U.S. Grand Prix practice.
Just three thousandths of a second separated the two at the end of the day, even if Hamilton ended up top of the timesheets in both sessions despite an apparent gearbox problem that sidelined him towards the end of the second.
The Briton, chasing his fifth win in a row and 10th of the season, set the morning pace with a time of one minute 39.941 seconds at the undulating Circuit of the Americas.
Rosberg, 17 points behind his team mate with three races to come worth a maximum 100 points, was 0.292 seconds slower than the 2008 world champion.
Hamilton then produced a 1:39.085 in the afternoon, a massive 1.1 seconds quicker than third placed Ferrari driver Fernando Alonso. The difference between the two was the same as that between Alonso and 16th placed Adrian Sutil.
McLaren's Jenson Button had been third fastest in the opening session with just 18 cars on track after Caterham and Marussia went into administration and failed to make the journey to Texas.
Mercedes, who have already wrapped up the constructors' title, can equal McLaren's 1988 record, set by Alain Prost and the late Ayrton Senna, of 10 one-two finishes in a season in Sunday's race.
Back then there were only 16 races, compared to the current 19.
ORDER SHIFTED
Toro Rosso's Russian rookie Daniil Kvyat, who graduates to the main Red Bull team next year in place of departing four times champion Sebastian Vettel, was fourth fastest in the morning ahead of Denmark's Kevin Magnussen in the other McLaren.
The order shifted after lunch however, with Red Bull's Australian Daniel Ricciardo fourth after a power unit problem hampered him earlier in the day and Brazilian Felipe Massa fifth for Williams.
Vettel will start Sunday's race from the pitlane after exceeding his allocation of power units.
However Red Bull principal Christian Horner assured the fans that the German, who was seventh fastest on Friday morning and then 18th after doing only 13 laps in the afternoon, would not sit out qualifying.
"We'll certainly be taking part in qualifying," he told the BBC. "But we have to be very careful with our mileage, he has to get through three race weekends with this unit."
Williams' Brazilian reserve Felipe Nasr completed 19 laps in the car Valtteri Bottas will race on Sunday and was eighth fastest in the morning session while Toro Rosso gave Dutch teenager Max Verstappen another run. He was 10th fastest.
Drivers also trialled a new 'virtual safety car' system, being tested as a means of reducing speeds electronically across the field when yellow flags are waved without introducing the real vehicle.
The system is being looked at following Frenchman Jules Bianchi's horrific Japanese Grand Prix accident. The Marussia driver remains critical in hospital a month on. (Editing by Toby Davis)