Mercedes drivers Valtteri Bottas and Lewis Hamilton outpaced their rivals to lead the opening practice session for this weekend's Russian Grand Prix in Sochi.
Bottas clocked in with a 0.211s margin over his teammate while Hamilton's fellow title contender Max Verstappen was third, 0.227s adrift from the leading Finn.
Ferrari's Charles Leclerc and Aston Martin's Sebastian Vettel completed the top five in FP1.
The weather forecast for Friday along the shores of the Black Sea were dire, yet the sun had pierced Sochi's clouds when the weekend's first practice session kicked off.
As a reminder, or perhaps an omen, Sochi is statistically a Mercedes bastion, with the Brackley squad undefeated in Russia since the country's inclusion in the F1 world championship in 2014, with Hamilton snatching four wins overall while Bottas has won twice in Sochi, including last year.
Ferrari's Carlos Sainz won the race to the green light, the Spaniard followed by Alpine's Esteban Ocon and Scuderia teammate Charles Leclerc, the recipient of an engine update this weekend.
As the bulk of the field head out on track and upon the conclusion of the usual installation laps, Bottas and Hamilton assumed an early lead, with the pair running on the hard compound.
But a soft-shod Verstappen, despite complaining about a "horrible recharge mode", leaped to the front with a 1m36.055s that cleared Bottas by 0.357s.
The Dutchman pressed on and improved his pace, but a switch to the soft tyre propelled Bottas to the top as the Finn lowered the benchmark to 1m34.427.
Hamilton and Leclerc - also on the softs - followed suit as the pair shuffled to P2 and P3, just ahead of Verstappen.
Lap time Improvements were few and far between as the session entered its second half. A purple second sector nevertheless pushed Verstappen up to third, but the Red Bull driver remained 0.227s off Bottas' fastest time.
Most drivers appeared to privilege long runs as the action unwound, an approach perhaps justified by tomorrow's wet day weather forecast which could allow for little race preparation running.
The session thus concluded with no changes at the front, with the Mercedes duo of Bottas and Hamilton leading proceedings and followed by Verstappen, Leclerc and Vettel who rounded off the top-five.
AlphaTauri's Pierre Gasly concluded his morning n P6, the Frenchman followed by Sainz, Norris, Perez and Alonso who completed the top ten for Alpine.
Among the runners in the second half of the field, Monza winner Daniel Ricciardo enjoyed a low-key start to his Russian Grand Prix weekend, clocking in P14, while Kimi Raikkonen's return to the fray with Alfa Romeo yielded a P15.
Further down the order, local hero Nikita Mazepin kicked off his home race weekend by outpacing Haas teammate Mick Schumacher as the pair bookended the field.