Lewis Hamilton fears overturning Sebastian Vettel and Red Bull's current domination in Singapore may prove difficult following the German's performance in second practice.
Hamilton set the ultimate pace in free practice one, but was seven tenths off Vettel in the second session - in which Fernando Alonso also finished above the Briton.
Hamilton said that the uncertainty over fuel loads meant there was some hope the gap to Vettel could be closed, but admitted he was not confident it could be overturned completely.
"Friday is really difficult to determine," the 26-year-old explained. "There is a big gap between us and Sebastian right now, and I suspect tomorrow there will be a big gap in qualifying unless there is a big difference in fuel loads.
"I definitely think getting them at the moment will be very difficult. Its not impossible, but with the pace he has right now it looks pretty tough. Fuel load and tyres can make a huge difference though, so I'm stilloptimistic.
"I don't know what Ferrari was running but I pray they have gone light too."
Hamilton said that while outright pace would be important, the key to the race could be making the tyres last, particularly with degradation proving so troublesome in practice.
"The key for victory is sheer pace," Hamilton said. "Positioning and qualifying are very important, and then after that looking after your tyres is really the key so we have to make some adjustments to see if we can try and reduce the sliding we have.
"Singapore is very, very tough on these tyres. I didn't get the first lap of my option tyre run and that's really when the tyres are at their best. When I came to the second attempt the rears just gave up, so when I attacked the corners they just slide.
"They get to a point where they just don't work any more and then you're done. It's about trying to reduce that so we've got to change the set-up a little bit."