Jenson Button thinks Red Bull will be hard to beat in qualifying after seeing Sebastian Vettel and Mark Webber lead the way in Friday practice.

The pair ended the first day of running in Korea three-tenths clear of anyone else, with Button setting the fifth best time on the combined timesheets.

The McLaren man was four-tenths of a second off the pace after the two sessions and despite being pleased with his race pace, the Briton said he didn't expect to be in the fight for pole.

"It's so tricky to get a good set-up around here," he said. "We're still not quite quick enough, but we're heading in the right direction. We've been making progress all day, although this morning's session was more about testing new parts and making sure everything worked correctly and less about choosing the right set-up.

"At the moment, it doesn't look like we're quite as quick as the Red Bulls - who'll probably be quite difficult to beat in qualifying - but our race lap-times and consistency both look good.

"I think the track has done most of its evolution already today - it's not going to re-set tomorrow - so the adjustments we've been making are looking positive. And there's still more to come."