Valtteri Bottas led Lewis Hamilton in a Mercedes one-two in first practice at the Russian Grand Prix.
He was 0.211 seconds ahead of Hamilton, with Red Bull's Max Verstappen third just 0.016secs behind his title rival, who the Dutchman leads by five points.
Ferrari's Charles Leclerc, who starts at the back of the grid after a penalty for using too many engines, was fourth.
Aston Martin's Sebastian Vettel and Alpha Tauri's Pierre Gasly completed the top six.
Verstappen has a three-place grid penalty after being adjudged "predominantly to blame" for causing his crash with Hamilton during the last race in Italy.
And Red Bull have said they are considering whether to fit his car with a new engine, which would mean he would join Leclerc at the back of the grid.
Verstappen needs to take another engine to have sufficient supply to make it to the end of the season, after one was damaged in his crash with Hamilton at the British Grand Prix in July. The only question is at which race the team will decide to take it.
As Red Bull believe Sochi is a track that favours Mercedes, they may consider that it is worth getting out of the way this weekend, especially as Verstappen has a grid penalty anyway.
Verstappen did only 13 laps in the session, compared to more than 20 for most other drivers, and the Dutchman's Red Bull ended it in the garage up on stands.
The session was held in warm sunshine at the Black Sea resort in temperatures around 19C but heavy rain is expected for qualifying day on Saturday.
It was a mostly incident-free hour around one of the calendar's most uninteresting circuits, apart from a couple of incidents from McLaren's Lando Norris.
The Briton ran wide at Turn 10 midway through the session and then, in a bizarre incident, clipped the wall on the inside as he rounded the right-hander on the entry to the pit lane at the end of the session, spinning and damaging his front wing.
Norris was eighth fastest, behind the Ferrari of Carlos Sainz, with the second Red Bull of Sergio Perez and Fernando Alonso's Alpine completing the top 10.
Sainz's car does not have the upgraded hybrid system that is in the new engine fitted to Leclerc's car, but the Spaniard will receive it - and the consequent grid penalty - later in the season.