Although he is confident of McLaren's race-pace, Jenson Button fears Red Bull will have the edge in qualifying in Korea.
The reigning World Champions claimed the 1-2 in Friday's second practice with Sebastian Vettel edging Mark Webber by 0.032s while Button was fourth, 0.387s adrift.
"They've got that four-tenths again in practice," said the McLaren driver.
"But I think they're going to be very, very difficult to beat in qualifying. They seem to have that strength back."
Those times, though, were set in qualifying trim with low fuel loads and on the supersoft Pirelli tyres whereas later, on heavy fuel loads, Red Bull did not have it all their own way.
"Our race-pace feels good, forgetting the lap times, our consistency feels good." Button added.
"There is still more work needed to fight with the Red Bulls here but I think we're going in the right direction."
As for the track conditions, the drivers had to contend with a very dusty Korean International Circuit in first practice.
The track, though, did improve as the day wore on although Button reckons it was as good as it is going to get by the end of the afternoon.
"I think it's done most of it [evolving]," he told Sky Sports News.
"Today it's really improved a lot. As long as we don't have rain or strong winds I think it's not going to reset tomorrow. So today's information is very useful for the weekend."
Vettel Leads Red Bull 1-2 In Practice Two