Lewis Hamilton was mystified as to why he was so far off the pace in Korean Grand Prix practice.
The McLaren driver, who was quickest in the first session, ended the day eighth fastest, 0.885 seconds behind Sebastian Vettel's pace-setting Red Bull.
Hamilton said: "We were quite quick in the first session but I don't really know what happened in the second.
"We were struggling a bit, just trying different things, and for some reason I wasn't able to get a good time."
Hamilton was 0.498secs slower than Button, who was fourth fastest behind Vettel, the German's Red Bull team-mate Mark Webber and Ferrari's Fernando Alonso.
"It was such a big difference from P1 to P2 and the car wasn't really that different. Just a small change on the rear ride-height which is nothing," he said.
"We just have to be calm and figure out which way to go. Jenson was not in a bad position so the car has some pace and we just need to extract it. I think we'll be OK."
Asked if he thought he could fight with the Red Bulls for pole position, Hamilton said: "I'm not sure. The Red Bulls seem very quick."
Button was more encouraged, especially by his pace on his race-simulation run with a high fuel load, when he was faster than the Red Bull and only marginally slower than Alonso and his Ferrari team-mate Felipe Massa.
Button said: "We have made good progress. Still not quick enough but I think we're going in the right direction.
"Red Bull have that 0.4secs again in practice. They are going to be very difficult to beat in qualifying. They seem to have that strength back.
"Our race pace seemed to be competitive but over one lap we are still lacking so it's an area we need to work on.
"But the direction we have gone with the car is positive so there is still more to come."
Alonso heads into the weekend with a narrow four-point lead in the championship following his failure to finish last Sunday in Japan, where Vettel won.
The Ferraris looked competitive on race fuel and Red Bull's closest challengers on outright one-lap pace, but Alonso said he was not sure where he stood.
"It's hard to say where we are compared to the others because we focused mainly on ourselves," the Spaniard said.
"We must wait until tomorrow to have a more precise picture of the situation."
Vettel said he was content with his team's progress.
"All in all we can be quite happy,' he said. "It looks extremely tight. We have to improve ourselves to match the others."