Charles Leclerc ensured Ferrari started the Russian GP weekend on top by setting the pace in Practice One, ahead of Red Bull's Max Verstappen.
Ferrari are looking for a fourth consecutive victory this weekend in Sochi and Leclerc went fastest in first practice with a 1:34.462, while Verstappen, who has a five-place grid penalty this weekend due to a new engine, was his surprise closest challenger - within a tenth of the Monegasque.
Sebastian Vettel, who ended his year-long wait for a victory last week in Singapore, was half a second off the leading pair in third after making errors on his flying lap in the other Ferrari.
"Leclerc is grabbing the Ferrari and extracting it to the limit," said Sky F1's Paul Di Resta. "Vettel is playing catch-up."
Direct comparisons between Ferrari and expected Russia rivals Mercedes, however, will have to wait as Valtteri Bottas and Lewis Hamilton failed to post a time on the quickest soft tyres, unlike Ferrari and Red Bull.
Bottas was seven-tenths off Leclerc on the medium compound, with Lewis Hamilton a tenth further back in fifth.
Mercedes are hoping to avoid a hybrid-era first this weekend: The five-time world champions haven't been beaten in four straight races since 2013.
Honda's worry, Ricciardo's crash
Alex Albon was sixth for Red Bull, and with all Honda-powered drivers taking engine penalties this weekend it was understandable that the rookie was concentrating on long-run pace in first practice.
But worryingly for Toro Rosso and Honda, Daniil Kvyat, who will start at the back of the grid with his new Power Unit, suffered a failure early on in the Friday morning session.
Toro Rosso boss Franz Tost told Sky F1 that it looked like a fuel issue, but that the team were still investigating.
Renault, looking to bounce back to the form they displayed in Monza, enjoyed a more positive session as Nico Hulkenberg and Daniel Ricciardo led the midfield in seventh and eighth - but they also had a late scare.
In the dying embers of the session, Ricciardo lost the rear of his car and slammed backwards into the barriers.
It remains to be seen how much work will have to be done ahead of second practice.
McLaren, fourth in the championship, were only 11th and 16th through Carlos Sainz and Lando Norris.
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