Sebastian Vettel insists there is room for improvement at Red Bull after ending Friday practice for the Korean Grand Prix in second place.

The defending champion goes into the weekend seeking to extend his championship advantage following a dominant success last time out in Singapore.

Although expected by many to be the man to beat, Vettel instead had to settle for second place in both Friday sessions behind the Mercedes of Lewis Hamilton.

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Speaking afterwards, Vettel admitted he was ready for a stern challenge from Mercedes through the remainder of the weekend, with Red Bull still having more to come.

"There is some room for improvement," he said. "It will be close with Mercedes; of course we're not sure what others were doing with their fuel loads today, but it seems close ahead of tomorrow's qualifying. I like the track here, it's a shame it's so far from Seoul, which is a great city, but it's good coming here as it's quieter than some other races, which means we can focus on our job."

Team-mate Mark Webber meanwhile was fourth fastest but said he was far from happy with the balance of his car ahead of a weekend where he will take a ten-place grid penalty.

"We need to work on the balance," he said. "It's more about understanding the tyres again and getting those working as well as possible. We need to work on both the short and long run balance, as obviously I'm going to be out of position after qualifying.

"We need to have a car which can be strong in the first sector, so we can get some overtaking done."