Mercedes' Lewis Hamilton set the pace in first free practice at the Korean Grand Prix on Friday as injury-hit Finn Kimi Raikkonen walked away from a late crash.
Hamilton, whose debut season with Mercedes is in danger of petering out after disappointing results in the past two races, was quickest with a time of 1min 39.630secs.
That was just 0.037 ahead of world championship leader Sebastian Vettel and 0.186 better than Vettel's departing Red Bull team-mate Mark Webber.
Lotus driver Raikkonen, fourth in the title race, was eighth quickest but the Finn spun off towards the end of the session and slammed into a wall.
The Ferrari-bound Raikkonen appeared to walk away unharmed but the incident will spark fresh concern over his long-term back injury which flared again last month in Singapore.
Mercedes driver Nico Rosberg was fourth quickest, ahead of McLaren's Jenson Button in fifth, followed by Ferrari's Fernando Alonso and Lotus's Romain Grosjean.
Sergio Perez of McLaren, Alonso's Ferrari team-mate Felipe Massa rounded out the top 10. Friday's second free practice session starts at 2:00 pm (0500 GMT).