By Alan Baldwin
AUSTIN Texas (Reuters) - Formula One championship leader Lewis Hamilton rounded off a hat-trick of fastest laps in U.S. Grand Prix practice on Saturday with Mercedes team mate and title rival Nico Rosberg 0.883 of a second slower after suffering a brake problem.
The Mercedes pair were again one-two on the timesheets after Hamilton went fastest in both of Friday's sessions, with Brazilian Felipe Massa third fastest for Williams on a clear and crisp Texas morning.
However, it was not a perfect session for the dominant team, with Hamilton spinning and Rosberg held up in the garage due to a glazed brake disc before then making some small errors that cost him a quick time.
Mercedes, who have already wrapped up the constructors' championship, were still the runaway favorites with Hamilton 1.107 seconds quicker than Massa.
Mercedes-powered cars filled the top four positions, with Valtteri Bottas fourth fastest for Williams.
Ferrari's Fernando Alonso was fifth, 1.620 seconds adrift of Hamilton's pace.
Hamilton leads Rosberg by 17 points in the standings with three races, and double points on offer for the finale in Abu Dhabi, remaining.
Red Bull's quadruple world champion Sebastian Vettel, who will start from the pitlane on Sunday after exceeding his engine allocation, clocked the slowest time of all as he focused on race setup rather than qualifying.
(Reporting by Alan Baldwin, editing by Ian Chadband)