Lewis Hamilton is concerned that he has "no hope" in the Brazilian Grand Prix if it stays wet.
Hamilton admitted that he had struggled all year with Mercedes' characteristics in the wet even though he traditionally revels in such conditions.
"Looking back at my career, in the rain that was when I could really make the difference, but in this car there is no hope for me," he said.
"I don't know what's going on. I'm just really, really struggling with the throttle mapping and all these kinds of things."
His team-mate Rosberg was fastest in both Friday sessions.
"Clearly, the car is quick enough because of what Nico was able to do so I really need to work hard," said Hamilton.
Although none of this year's grands prix have been wet so far, Hamilton said his experience in damp practice sessions and the partially wet Melbourne qualifying had been disconcerting.
"I'm just struggling to keep it on the track," he said.
"It was the same in Australia and I can't remember where else it has been raining, but it has always been the same."