Jenson Button believes McLaren needs cooler temperatures over the weekend to have a hope of being competitive in the Malaysian Grand Prix.
The Formula 1 constructors' championship leading team struggled at Sepang on Friday, with Button and team-mate Kevin Magnussen ending the afternoon eighth and 12th.
"We're finding it tough out there today, especially in this heat," said Button.
"We're very weak in high-speed corners so a lot of work is needed.
"Hopefully the weekend is going to be cooler than it is today. It's supposed to be in qualifying and the race. We need it to be to be relatively competitive.
"We've found it tough on the long run."
With qualifying and the race both scheduled for later in the day than the practice sessions, Button is optimistic that conditions might be more in McLaren's favour.
"I think we're going to get that, the car was working better this morning in cooler conditions, so hopefully that would help us," he said.
"There are a few things in terms of set-up we need to do but we don't have the pace right now, which is a surprise actually."
McLaren has brought an upgrade package to Malaysia that team chief Ron Dennis predicted would be worth half a second per lap.
Button reckons whatever gains McLaren had managed were currently masked by its lack of harmony with the temperatures.
"I think it always looks bigger than it is when you change the nosebox," he said of the upgrade.
"They're going through the data still and hopefully it is giving us something. I think it probably has, which is good, but then the high temperatures and high speed corners are really hurting us at the moment and that is an area where we are weak.
"I think when you have issues with high speed it's always exaggerated when it's so hot."