Sebastian Vettel led a Red Bull one-two this morning in the second free practice for a South Korean Grand Prix that could send the Formula One world champion back into the overall lead for the first time since the fifth round.
The 25-year-old German, four points behind Ferrari's Fernando Alonso with five races remaining, set the fastest lap of a clear and sunny day in the afternoon session with a best time of one minute 38.832 seconds.
Australian team mate Mark Webber, who lined up with Vettel on the front row of the grid in Japan last Sunday in a race that the German won from pole, was a mere 0.032 slower after being third fastest in the morning.
Alonso was third quickest in the second session, 0.328 off Vettel's pace but with a more competitive looking Ferrari.
McLaren's Lewis Hamilton had kept Alonso off the top in the morning with a best effort of 1:39.148. The 2008 champion was eighth fastest after lunch at the dusty circuit some 400km south of Seoul.
Hamilton needs to deny Vettel, last year's winner in Korea, a third win in a row on Sunday to get back into a title fight that looks increasingly like a duel between Spaniard Alonso and the Red Bull champion.
Lotus's Kimi Raikkonen, absent from the first two Korean Grands Prix due to his switch to rallying, had his first taste of the circuit and was 11th and 10th respectively in the two sessions.
The Finn was trying out a new 'Coanda' exhaust system that his team hope will bring big benefits in the closing races as they battle Ferrari for third place in the constructors' standings.
With the circuit little used for most of the year, drivers took their time setting fast laps when the first session opened.
Alonso, who was pitched out at the start of last Sunday's Japanese Grand Prix, was an early pacesetter with Hamilton faster towards the finish.
McLaren's Jenson Button was 10th and fourth fastest at a track he described on Thursday as: "Monza for the first 45 seconds of the lap, then a mid-high-speed section, quite flowing, and then you've got driving around your living room at the end of your lap."
Sauber's Sergio Perez, the Mexican who joins Button at McLaren next year as Hamilton's replacement, stopped on the track in the afternoon after the halfway mark in the session. The Swiss team said it was an actuator problem and the engine was not damaged.
Finland's Valtteri Bottas took Brazilian Bruno Senna's place for the first session at Williams while French reserve Jules Bianchi came in for Nico Hulkenberg at Force India and was 13th.
Other less familiar faces for the morning were Dutch tester Giedo van der Garde for Russian Vitaly Petrov at Caterham and Spaniard Dani Clos replacing Indian race regular Narain Karthikeyan at HRT.