Russian GP: Ferrari pace 'more robust' than Mercedes', says Wolff

Ferrari topped both Sochi practice sessions on Friday, with Kimi Raikkonen fastest in the morning and championship leader Sebastian Vettel in the afternoon.

Vettel led a one-two in second practice, with Valtteri Bottas the best of the Mercedes drivers, 0.670 seconds off the pace with Lewis Hamilton just behind.

Both of Wolff's drivers' fastest times came several laps into a run, and they also had off-track excursions on a "difficult day" trying to set representative times.

"You could see you needed a couple of laps to put the tyre in the window, and you needed to have clean laps, then it's easier," Mercedes boss Wolff said.

"And if you haven't got the clean laps, you won't be putting it in the right spot.

"The wider the window is, and the easier you put the car into the tyre window, the faster you go.

"The more narrow it is, the more difficult it is to hit the window, and this is what we are seeing at the moment.

"The Ferrari seems to be a car that is pretty robust in a wider window temperature than ours, and it doesn't matter whether it's Bahrain, where it's very hot, or Sochi where it's cold.

"It's just a very good car."

Ferrari has won two of this year's first three races, with Vettel's victory in the season-opener in Australia the team's first since winning in Singapore in 2015.

Mercedes never found a way to make its car work on that street circuit that weekend, but Wolff believes it is not on for a repeat in Russia.

"There's a pretty good indication what's happening, and what they have been doing, so it's all relaxed until now," he said.

"In Singapore 2015 we were the only team out of position, and we just couldn't make the tyre work.

"What you can see is there is one team that is extremely fast today, for whatever reasons.

"There are pretty good clues."