Sebastian Vettel set the fastest time in Friday's practice for the Korean Grand Prix, edging Red Bull teammate Mark Webber and Formula One championship leader Fernando Alonso.
Vettel, who has won the previous two races to move within four points of Ferrari's Alonso in the drivers' standings, was 0.032 seconds faster than Webber at the Yeongam circuit. Alonso was 0.328 seconds off the pace.
McLaren driver Jenson Button was fourth followed by Mercedes' Michael Schumacher and Ferrari's Felipe Massa. Button's teammate Lewis Hamilton, who was fastest in the morning session, struggled to find speed on the softer tires in the afternoon and finished eighth behind Mercedes driver Nico Rosberg.
Force India's Nico Hulkenberg was ninth while Lotus' Kimi Raikonnen - third in the championship standings - was 10th.
Many drivers struggled to find grip on the dusty track. Hamilton went off the track late in the afternoon session as did Williams driver Bruno Senna, who finished 12th.
Sergio Perez experienced engine trouble with his Sauber which ground to a halt midway through the session. Race marshals had to roll the car off the track at Turn 12 and Perez was done for the day.
The Mexican driver, who will move to McLaren next year, had to settle for 15th place after 11 laps.
Webber and Vettel swapped the top spot for almost the entirety of the second practice session.
Friday's result was an early indication the Red Bulls will be tough to beat on the weekend when Vettel will be aiming to overtake Alonso in the drivers' standings and the team tries to extend its lead in the constructors' championship.
Vettel won the Korean GP last year and arrives in Korea having won the previous two races in Japan and Singapore to erode Alonso's lead with five races left.
Earlier, McLaren driver Lewis Hamilton set the fastest time in Friday's opening practice for the Formula One Korean Grand Prix.
Hamilton edged championship leader Fernando Alonso of Ferrari by 0.302 seconds while Red Bull driver Mark Webber was third at the Korean International Circuit.
Alonso leads Red Bull's Sebastian Vettel by four points with five races left. Vettel was fifth-fastest in the first of two practice sessions Friday, behind Ferrari's Felipe Massa.
Alonso, who crashed out at the first corner of the previous Japanese GP, led for most of Friday's early session but Hamilton replaced the Spaniard at the top with four minutes left after a lap of 1 minute, 39.148 seconds.
The Mercedes-bound Hamilton will be looking to re-ignite his championship hopes in Sunday's race. The Briton is in fourth place in the standings, 42 points behind Alonso.
Mercedes driver Michael Schumacher was sixth followed by teammate Nico Rosberg. Lotus' Romain Grosjean was eighth while Force India's Paul di Resta and McLaren's Jenson Button rounded out the top 10.
Lotus driver Kimi Raikkonen - third in the championship standings - was 11th on the timesheets.