Lewis Hamilton was fastest by a large margin as Mercedes' closest rivals Ferrari hit trouble in second practice at the Russian Grand Prix.
Hamilton was 0.652 seconds clear of Ferrari's Sebastian Vettel, whose car stopped on the track with an electrical failure halfway through the session.
Nico Rosberg was third, 0.867secs off team-mate Hamilton, but did not set a representative lap on the faster tyre.
The German was just 0.14secs behind Hamilton on the slower 'soft' tyre.
Sunday's Russian Grand Prix - which is live on the BBC Sport website and BBC Radio 5 live Sports Extra - begins at 13:00 BST. Qualifying is at 13:00 on Saturday.
Hamilton in control - and needs to be
It was exactly the start to the weekend Hamilton would have wanted. The Englishman arrived in Sochi on the Black Sea coast determined to notch his first win of the season and peg back Rosberg's already sizeable advantage in the championship.
Rosberg is leading Hamilton by 36 points after winning all three races so far. The world champion has had a troubled start to the season and has finished second, third and seventh.
Mercedes appear to be in a league of their own so far this weekend.
The trend of the season so far has been Ferrari have tended to look competitive in practice only for Mercedes to extend their advantage in qualifying.
But in Russia, Mercedes have been comfortably quicker from the start of running - with well over half a second of advantage over the red cars.
And Hamilton showed highly impressive pace on his race-simulation run later in the session, comfortably clear of anyone else, including Rosberg.
"It's been a constructive day to start the weekend," he said. "I think we're looking strong here - but we need to keep working hard if we are to try and stay ahead of the Ferraris."
Electrical storm at Ferrari
In addition to an apparent lack of pace, Ferrari will be concerned about another reliability failure, especially as they have brought a new, upgraded engine design to this race.
The team have suffered two in-race engine failures so far this season and the impression is forming that they have pushed their reliability too close to the edge in an attempt to close the performance gap to Mercedes.
Vettel's team-mate Kimi Raikkonen was fourth fastest overall, but more than 0.5secs slower than the German.
"The feeling was all right," said Vettel, who must take a five-place grid penalty on Sunday after being forced into an unscheduled gearbox change.
"The balance was not quite where I wanted it to be, but the circuit should come our way. It was quite slippery. Lacking a bit of information from my side but other people did their homework and hopefully we can copy that a bit."
Behind Mercedes and Ferrari, Red Bull's Daniel Ricciardo was fifth quickest, just 0.101secs ahead of Williams driver Valtteri Bottas, with the second Red Bull of Daniil Kvyat only 0.008secs further adrift.
Behind them, there was some encouraging pace from the McLaren-Hondas, with Jenson Button eighth and Fernando Alonso 10th, sandwiching the second Williams of Felipe Massa.
Next in line were the Toro Rossos, Carlos Sainz edging Max Verstappen, and the Force Indias, with Nico Hulkenberg pipping Sergio Perez.
The new Haas team - who impressed in the first two races in Australia and Bahrain - were struggling, with Romain Grosjean and Esteban Gutierrez 16th and 17th respectively, and the Franco-Swiss saying "something is not working on the car" since the Chinese weekend a fortnight ago.
Aeroscreen 'definitely drivable'
In the morning session, Ricciardo gave the first on-track test to the 'aeroscreen' cockpit head protection design, which Red Bull are hoping will be adopted next year instead of the 'halo' device tried by Ferrari pre-season.
The Australian said: "The vision seemed OK; the first impressions were fine. It was definitely drivable. I would not say you are hindered any more than we are now with visibility."
Governing body the FIA has made it clear it intends for one of the devices to be used next season.
Ricciardo is among the vast majority of drivers who back the move - despite criticism in some quarters - because of the number of accidents in recent years in which drivers have been killed or injured by head impacts.
"The talk has been about the open cockpit and that is what people know F1 for, and that is fair enough," he said.
"It would be great to keep it that way but obviously with accidents that have happened, especially more recently, I think at least not to explore this route seems a bit disrespectful.
"If this becomes the norm, I think everyone will get used to it. I don't think it is as bad as some people are saying."
McLaren driver Alonso re-iterated his support for some from of head protection to be introduced.
"It is a must that it will come from next year," he said. "It is a must for safety - we don't need heroes in the sport right now.
"We don't want anyone getting hurt in the future if there is a solution in place, and it seems there can be a solution so let's introduce it."
Team-mate Button praised design of the 'aeroscreen' device, saying: "I prefer the look of it. I think it looks better than a normal F1 car."