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.
Mercedes have won all of the three previous races held at the Sochi Autodrom and led every lap, and Vettel said he felt Mercedes had been "sand-bagging" -- deliberately holding back to lure Ferrari into a false sense of superiority.
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."
The Briton Hamilton, seven points behind Vettel in the early championship standings in what is shaping up to be a two-way fight for the world title, hit back saying: "We never sandbag.
"There's no benefit to sandbagging, ever. I think Ferrari has done in the past, but there is zero benefit for us to do so.
"We're trying to get the car to the optimum balance-wise and today's generally been a tricky day.
"I don't know why he would suggest that, it's just Sebastian trying to blow smoke our way. Really, those guys are just very quick."
Raikkonen enjoyed his revival in form following suggestions that his paymasters at Ferrari have been underwhelmed with his performances in the three races so far this season.
"I'm happy that things ran smoothly with no issues," the Finn said.
"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."
- Red Bulls off pace -
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.