Sebastian Vettel had the fastest time in Friday's practice for the Korean Grand Prix, edging Red Bull teammate Mark Webber and Formula One leader Fernando Alonso.
Vettel, who has won the previous two races to move within four points of Alonso in the drivers' standings, was 0.032 seconds faster than Webber at the Yeongam circuit. Alonso, who drives for Ferrari, was 0.328 seconds off the pace.
"This afternoon (the track) was quite slippery to start with," Vettel said. "But then I think it got a little bit better so, all in all, the car felt all right and now we see what we can do for tomorrow. I think we have to improve ourselves to match the others."
Alonso, who crashed out at the first turn of the Japanese GP after starting sixth on the grid, will be looking for better results in qualifying Saturday.
"It's hard to say where we are compared to others," Alonso said, "because today we focused mainly on ourselves, working this morning on a comparison of various ideas relating to car configuration and, in the afternoon, on the tires."
McLaren driver Jenson Button was fourth, followed by Mercedes rival Michael Schumacher and Ferrari driver Felipe Massa. Button's teammate, Lewis Hamilton, was fastest in the morning session but struggled to find speed on the softer tires in the afternoon and finished eighth behind Mercedes driver Nico Rosberg.
"The first session was great, I don't really know what happened after that," Hamilton said. "I'm sure we'll find out what the problem is."
Force India driver Nico Hulkenberg was ninth while Lotus rival 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, 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 entire second practice.
Friday's result was an early indication that 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.
Button said the Red Bull drivers are becoming stronger as the season winds down.
"They are going to be difficult to beat in qualifying," Button said. "They seem to have that strength back. Our race pace seemed to be competitive but over one lap we are lacking so that's an area we need to work on."