Valtteri Bottas secured a surprise pole position at the Russian GP as he pipped team-mate Lewis Hamilton to head a Mercedes one-two.
It was a sublime qualifying performance from Bottas, who is without a win this season, and he outpaced Hamilton by a tenth of a second after posting a new Sochi track record, a 1:31.387.
But crucially for Hamilton, he will line up one place ahead of title rival Sebastian Vettel on the grid as an off-colour Ferrari finished half a second off Mercedes.
"It was a nice lap," said Bottas, whose only previous F1 2018 pole came in Austria. "It feels good. I am very happy but it's only the first step of this weekend."
Where has Ferrari's speed gone?
Bottas outpaced Hamilton on both of his Q3 laps on hypersoft tyres as the championship leader struggled in the final portion of an intriguing session, making a mistake on his last lap.
"Big congratulations to Valtteri - he did the better job," admitted Hamilton. "My last two laps weren't so special but at least we are still in the fight for the race."
Kimi Raikkonen was fourth for Ferrari, who were at least spared a battle with Red Bull as neither Max Verstappen or Daniel Ricciardo set laps due to their grid penalties.
Ferrari half a second behind
The Scuderia will be alarmed by their major deficit to Mercedes, who have immediately made the most of their upgraded package in Russia.
"It's been a tricky one. Why the big gap? It is a surprise," said Vettel.
Red Bull's decision not to enter Q3 opened up a battle for fifth place for the midfield, and that was won by Haas' Kevin Magnussen.
Esteban Ocon was sixth for Force India, just six thousandths of a second ahead of Sauber's Charles Leclerc, who sealed his best ever qualifying position.
Five drivers go missing
There was a bizarre middle portion of the session, meanwhile, as Q2 was effectively meaningless.
Verstappen and Ricciardo didn't post a time as Red Bull are taking engine penalties in Sochi, much like Pierre Gasly and Toro Rosso.
Renault didn't have that same excuse but still opted against sending Nico Hulkenberg or Carlos Sainz out, preferring to have free choice with tyre strategy from outside the top-10.
That decision came after drivers struggled at the start of the Singapore GP on the quickest, but less durable hypersofts, which the midfield runners will be using here in Russia.
McLaren had both Fernando Alonso and Stoffel Vandoorne eliminated from Q1 for the third time this season, while fellow former greats Williams also couldn't get a car through.
Sergey Sirotkin was set for an improvement before spinning on his final lap, while team-mate Lance Stroll has endured a painful weekend.
Sky Sports F1 is the only place to watch every Formula 1 Grand Prix, qualifying and practice session live in 2018. Get Sky Sports F1.