Felipe Massa has suggested Red Bull Racing may be out of reach this weekend given the team's practice pace on Friday at Suzuka.
Although championship leader Sebastian Vettel finished down in third position, the German's long-race pace was impressive, lapping in the 1m37s, a time Massa labeled as "too quick."
"That's quick, isn't it? 37 was a little bit too quick," said Massa. "We need to see for sure if you go to the long run with 20 kilos less or 20 kilos more, or 30 kilos – maybe some were running with 100kg and some with 140kg, you can see a big difference.
"We will wait and see in the race, which is the right time to have a clear picture."
Massa admitted he was unable to go through the fast 130R corner with his DRS open like rival Red Bull: "It is not easy for our car. Maybe tomorrow there will be a bit more grip and we can see if it is going to be better."
The Ferrari driver, fifth quickest today, said he was pleased with his session, but admitted the real gap to the front will only be seen tomorrow.
"I think it was OK. We only need to wait until tomorrow to see the real gap between the cars but I think it was a good Friday anyway, the car feels good, and I hope better than the last races.
"So for sure the race here will not be easy in terms of tire degradation. It is a track that uses a lot of tires as well, so it is something that we need to be aware of with the right strategy as well."