Formula One: Mercedes still the team to beat in Russia - Vettel

Championship leader Sebastian Vettel believes his Mercedes rivals remain the team to beat at this weekend's Russian Grand Prix despite Ferrari convincingly topping Friday's practice times.

The four-time world champion German and his Ferrari team-mate Kimi Raikkonen were first and second fastest at the end of an uneven day's action in Sochi, the Finn having been fastest in the morning.

The Ferraris were more than six-tenths clear of third-placed Valtteri Bottas in the leading Mercedes with team-mate and three-time champion Lewis Hamilton admitting he struggled to wind up a disappointing fourth.

But Vettel said: "It's a circuit that suits them so they will be strong tomorrow. I think last year people expected Williams to be fastest after Friday, if I remember right, and it turned out Mercedes was the fastest.

"That's how sometimes you can be misled and I think there's a lot of things we can play with in the car -- fuel loads, engine modes...

"This looks like a Mercedes track. For sure they didn't show everything today and they didn't get a lap together.

"So the gap you see, it's artificial."

Mercedes have won all of the three previous races held at the Sochi Autodrom and led every lap.

Hamilton, who is seven points behind Vettel after three races, admitted he had endured a challenging day as he bids to keep his title bid on track.

"It was a bit of a difficult day for us," the Briton said.

"We managed to complete everything that we needed to do on our runs, but in terms of the balance of the car, the Ferrari seemed very, very fast on the long runs.

"So we need to work out how we can improve our pace, but there's still everything to play for.

Raikkonen, enjoying his revival in form, said: "I'm happy that things ran smoothly with no issues.

"We had quite a good feeling with the car straight away so it's always more fun like that. So far it's fine and tomorrow we have to make it a good day and go day by day."

Dutch teenager Max Verstappen was fifth-fastest despite stopping on the circuit following a loss of power, ahead of his Red Bull team-mate Daniel Ricciardo.

The Australian admitted that the leading duo of teams had blown Red Bull away with their pace.

"In terms of us here this weekend, we know this is what we have got, so we are trying to do what we can with the current car and package," he said.

"We have to try to target the top five. When I say that, it is behind Ferrari and Mercedes. To try to be the next best after them is our realistic target."

Brazilian Felipe Massa, showing some of his best form again this season after taking a U-turn on retirement, was seventh for Williams ahead of Nico Hulkenberg of Renault, Dane Kevin Magnussen of Haas and Mexican Sergio Perez of Force India.