Nico Rosberg topped the times ahead of his Mercedes team-mate Lewis Hamilton in Friday's rain-hit opening free practice session for this weekend's Brazilian Grand Prix.
The 28-year-old German clocked a best lap time of one minute and 24.781 seconds at the spectacular Interlagos circuit early in the 90 minutes' action and, as the rain intensified, it proved to be untouchable.
Briton Hamilton, the 2008 world champion, was second in 1:25.230 ahead of four-time world champion German Sebastian Vettel of Red Bull and the 2009 champion Jenson Button of McLaren.
Two-time champion Fernando Alonso of Ferrari was fifth-quickest ahead of his friend, retirement-bound Australian Mark Webber, who competes in his final race with Red Bull this weekend.
Mexican Sergio Perez, in his last weekend with McLaren, was seventh ahead of impressive 19-year-old Russian rookie Daniil Kvyat, running for Toro Rosso ahead of his move to replace Red Bull-bound Australian Daniel Ricciardo next season.
Finn Heikki Kovalainen, who returned for Lotus in Texas last weekend, was ninth ahead of highly-rated German Nico Hulkenberg of Sauber on a morning that gave few clues to the action ahead.
Several cars slithered, slipped or spun in the tricky conditions, including Hamilton and Mexican Esteban Gutierrez of Sauber, leaving the teams with few lessons learned, but hoping that the track would dry out before the second session in the afternoon.