Felipe Massa claims the sweltering heat in Germany helped Red Bull close the gap on Mercedes but expects the latter still has the advantage.
Friday's practices at the Hockenheimring took place under full sunshine with the track temperature climbing to a staggering 58'C in the afternoon session.
That practice once again resulted in a Mercedes 1-2, however, the Red Bulls weren't far behind with Daniel Ricciardo just 0.102s off the pace.
Massa reckons it was the heat that allowed the Bulls to close right up.
"I think the temperature maybe helps to be honest," he told Crash.net.
"They have a lot of downforce in the car so for sure they lose a little bit of speed but maybe they use the tyres a little bit better. Even on the supersoft at the end of the lap many teams were losing a bit of grip so maybe they use less.
"Anyway, Red Bull is a quick car, independent of the track they are going to be competitive. I still believe Mercedes is the best car in the field but let's wait and see tomorrow after qualifying."
As for his own team's prospects come Saturday's qualifying, the Williams driver, who was sixth quickest on Friday, says they will do everything they can.
"It's not really easy to be sure you're going to have the car to fight for pole position but we need to try everything we can."
He added: "It's pretty hot but honestly I was more happy with the supersofts compared to the softs.
"Sometimes things happen that you don't understand, but it's not just me; I saw some other cars going longer on the supersofts and being better on the supersofts compared to the softs.
"So we need to be ready for everything and try to use these tyres the best way we can in the race. If it's dry anyway."