Red Bull may have struggled to match Mercedes during practice for the Japanese Grand Prix but Nico Rosberg remains wary of their threat.
Nico Rosberg was pleased to top both the timesheets in both practice sessions on Friday ahead of the Japanese Grand Prix but remains cautious about the threat posed by Red Bull.
The Formula One championship leader was fastest in FP1 and FP2 at Suzuka, eclipsing his Mercedes team-mate Lewis Hamilton on both occasions.
Ferrari showed an improved performance to finish ahead of Red Bull through Sebastian Vettel's third-place finish in the opening session, with Raikkonen maintaining their advantage in FP2.
Max Verstappen showed good form to finish fourth in the second session, but his team-mate Daniel Ricciardo was surprisingly 12th after problems encountered by Esteban Gutierrez's Haas prevented him from setting a qualifying simulation lap.
However, both Red Bulls showed good pace in their long runs and their performances left Rosberg wondering how close they will be as the weekend goes on.
"It's been a case of finding my way today," said the German. "It began with quite a lot of under-steer and we had to work through the setup in order to find a better balance on the car.
"In the afternoon, the medium tyre didn't seem as strong as the hard as we saw in Malaysia. However, on the soft tyre we showed strong one-lap pace, the balance felt good and the track was really exciting to drive.
"It looks like we're ahead of the Ferraris but it remains to be seen as to where we are compared to the Red Bulls."
Hamilton finished the day pleased with his two second places, and the Briton hopes a few tweaks can make him even more competitive.
"It's been a really good day with no problems on the car, which is great," he said. "We managed to get plenty of laps in but there's still some work to be done overnight in order to find more pace.
"That's a work in progress and hopefully we'll be going into Sunday in good shape."