McLaren's Lewis Hamilton pipped Ferrari's Fernando Alonso to fastest time in first practice at the Korean Grand Prix.
The two men dominated the timesheets all session, with the Ferrari narrowly ahead of the McLaren for much of it, before Hamilton nipped ahead at the end.
Red Red Bull's Mark Webber and Sebastian Vettel were third and fifth, sandwiching the Ferrari of Felipe Massa.
The Mercedes of Michael Schumacher and Nico Rosberg were sixth and seventh.
Alonso set the pace as soon as he emerged out on to the track, initially moving a second clear of the field.
The Spaniard set a one minute 39.857 second lap, and then lowered it by 0.4secs to leave the mark at 1:39.450.
He was on at least two other laps that started faster than his best only to abort them later on - one after a big slide at Turn 14 in the 'street' section of the track towards the end of the lap.
His pace suggested the Ferrari team might be trying to work more towards qualifying at this race following the loss of Alonso's cushion in the world championship as a result of his retirement in Japan and Vettel's victory.
BBC F1 technical analyst Gary Anderson said: "Alonso is really pushing the car, and I think that gives us a clue that they are trying to put the car on the limit and not see how far the tyres will go for a race run."
Hamilton then closed up on Alonso with a one-off lap of 1:39.475, before beating the Spaniard in the final five minutes with a lap of 1:39.280 and then lowering his mark again with his final lap of 1:39.148.
BBC F1 commentator Ben Edwards said: "It looks once again as it if it could be all three of the big teams fighting for pole position and the win. It could be a very exciting and closely fought race, which is just what we want at this point in the championship."
Lotus driver Kimi Raikkonen, third in the championship ahead of Hamilton, was trying out a new exhaust design which he hopes will give extra life to his title challenge.
The Finn ended the session 11th fastest, behind his team-mate Romain Grosjean, Force India's Paul di Resta and McLaren's Jenson Button.
A number of drivers were caught out by the slippery circuit, which is virtually unused for much of the year, and several ran wide at corners having misjudged their braking points.