The German lapped the anti-clockwise circuit with a best time of one minute 24.781 seconds, set early on before conditions became more treacherous, with team mate Lewis Hamilton second quickest in 1:25.230.
The times were largely meaningless, with Red Bull's quadruple world champion Sebastian Vettel leaving it late in the 90 minute session to set a timed lap but ending up third on the timesheets.
All three had spins and off-track excursions during the course of the morning.
Vettel had earlier tried out Pirelli's slick 2014 development tyre, completing a cautious lap with sensors on the car before being told by his race engineer that they had all the data they needed and to return to the pits.
The German can equal compatriot Michael Schumacher's 2004 record of 13 wins in a season as well as Italian Alberto Ascari's 60-year old record of nine wins in a row if he ends the season with a victory on Sunday.
Both the drivers' and constructors' titles have been won already by Vettel and Red Bull, with the main focus of the weekend the battle for second place between Mercedes and Ferrari as well as various farewells.
Mercedes are 15 points ahead of Ferrari with Lotus a further 18 behind.
McLaren's Jenson Button was fourth quickest ahead of a weekend that could see his team's season confirmed as their worst in decades, without a single podium finish to their credit for the first time since 1980.
"I'm really struggling to get heat in the tyres," Button told his team over the radio early in the session.
Ferrari's Fernando Alonso was fifth ahead of Red Bull's Australian Mark Webber, gearing up for his last grand prix before heading off for a new future in endurance racing with Porsche.
Russian teenager Daniil Kvyat replaced Australian Daniel Ricciardo, the man he is replacing full time next year, at Toro Rosso for the first session and was eighth in his second involvement in a grand prix weekend.
Paul Di Resta also sat out the morning at Force India, with British reserve James Calado in his car.
(Writing by Alan Baldwin in London, editing by Justin Palmer)
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