Championship leader Sebastian Vettel reconfirmed Ferrari's resurgence by topping the times ahead of his team-mate Kimi Raikkonen in Saturday morning's final free practice for the Russian Grand Prix.
The four-time champion German led the way with a best lap in one minute and 34.001 seconds to wind up three-tenths quicker than the 2007 champion.
Ferrari's demonstration of speed and consistency left defending champions Mercedes panting in pursuit in third and fourth places.
Valtteri Bottas was unable to match his fellow-Finn, but finished ahead of three-time champion Briton Lewis Hamilton who struggled again and was half a second off Vettel's pace.
The outcome suggested that Vettel and Ferrari are poised to end Mercedes' domination at the Sochi Autodrom where they have reigned supreme in all three previous events.
Dutch teenager Max Verstappen finished fifth for Red Bull ahead of Brazilian Felipe Massa of Williams, Australian Daniel Ricciardo in the second Red Bull and German Nico Hulkenberg of Renault.
Spaniard Carlos Sainz was ninth for Toro Rosso and Dane Kevin Magnussen 10th for Haas.
On a warm and sunlit morning in the Black Sea resort, with an air temperature of 23 degrees Celsius and track of 34, home hope Daniil Kvyat was first out for Toro Rosso.
His first top put him top, but then he spun. Raikkonen and then Vettel clocked laps that lifted Ferrari clear, the German by six-tenths until Hamilton responded after 25 minutes.
Following his struggles on Friday, it was a fillip for Mercedes to see him record 1:34.828, one-tenth faster than Vettel, although the times were all slower than Friday.
Bottas improved to third, two-tenths down before Verstappen went fourth, ahead of Raikkonen, raising optimism for Red Bull.
Bottas then went top before Raikkonen, and then Vettel, and Raikkonen again, re-established Ferrari's supremacy to provoke glum expressions on the Mercedes pit wall.
Knowing a Ferrari front row was in prospect, Mercedes faced, arguably, their greatest challenge in four years as the Italian chassis demonstrated superior agility and consistency, notably at the final corner.