Lotus driver Kimi Raikkonen suffered a heavy crash as Lewis Hamilton set the pace for Mercedes in first practice at the Korean Grand Prix.
Raikkonen lost control of his Lotus through the penultimate corner and smashed into the barriers on the outside of the circuit.
The Finn, who suffered back pain at the last race in Singapore, appeared unhurt.
Red Bull's Sebastian Vettel was second and title rival Fernando Alonso sixth.
Vettel was just 0.037 seconds slower than Hamilton, with the Ferrari driver 0.744secs off the pace.
The Spaniard is 60 points behind Vettel heading into the race with 150 points available in the remaining six races.
Red Bull's Mark Webber was third fastest ahead of the second Mercedes of Nico Rosberg, with McLaren's Jenson Button fifth.
Raikkonen - who was eighth fastest, just behind team-mate Romain Grosjean - lost control in the penultimate corner, a right-hander, and the car spun through the left-hand kink that followed and into the tyre wall.
Lotus trackside operations director Alan Permane said there had not been a car failure and that the team would have no problems repairing it for the second session, which starts at 0600 BST. Raikkonen's back was causing him no problems, he added.
Lotus have both cars set up in long-wheelbase trim, a specification last tried - but not raced - at the Italian Grand Prix a month ago.
After a slow start it was a relatively eventful session. Several drivers spun or ran wide at the tricky Turn 10 right-hander, among them Hamilton and Force India's English reserve driver James Calado, who ended the session 18th fastest as he stood in for Scot Paul Di Resta.
Marussia reserve driver Rodolfo Gonzalez, standing in for Jules Bianchi in the first session, crashed early on and broke his car's nose cone.
The team managed to repair the car and the Venezuelan was able to get out again with 20 minutes of the session remaining. He ended it 2.71secs slower than team-mate Max Chilton.