Lewis Hamilton finished final practice for the Azerbaijan Grand Prix in third as his championship rival Sebastian Vettel was struck down by a mechanical issue.
Mercedes' Valtteri Bottas topped the order at the Baku City Circuit after he edged out Ferrari's Kimi Raikkonen by just 0.095 seconds with Hamilton, only 10th in practice on Friday, four tenths further back.
But for Vettel, who heads Hamilton in the title race by 12 points heading into the eighth round of the season, his morning was plagued by a technical problem.
The four-time champion was hauled back into the Ferrari garage with just half of the one-session completed, and he did not re-emerge.
Vettel's issue is yet to be diagnosed by the Italian team, but it will come as a hefty blow to the championship leader ahead of qualifying later on Saturday.
Vettel was not the only front runner to hit trouble with Red Bull's Max Verstappen, fastest in both practice sessions here on Friday, parking his Red Bull with 10 minutes remaining.
"There's something wrong with the engine," the teenager said before coming to an abrupt halt.
Jolyon Palmer is a man under pressure this weekend following his pointless start to the new season, and the Englishman suffered an engine failure in the opening moments of Saturday's morning running.
Palmer's Renault engine momentarily caught alight before the track marshals put the blaze out.
Fernando Alonso will start Sunday's race from the back of the grid following a series of changes to his Honda engine.
He will serve a mammoth 40-place grid drop, while his McLaren team-mate Stoffel Vandoorne has been penalised by 30 places following a raft of new parts on his engine, too. Alonso finished 14th on Saturday morning.
Elsewhere, Daniel Ricciardo was fourth for Red Bull ahead of Force India's Esteban Ocon with Verstappen in sixth.