Increasingly heavy rain caused a litany of incidents and proved particularly troublesome to the regular rally winners, most of whom were running late in the start order and therefore hit the worst of the stage conditions on the sodden stage two in particular.
While the factory Citroens and Fords floundered, M-Sport Ford man Tanak and his Adapta counterpart Ostberg burst to the front.
Tanak held a tiny 0.1s lead after stage two, with Ostberg then edging into a 2.7s advantage by the end of stage three.
Neither had a totally straightforward morning either: Tanak overshot a braking point and slid off briefly on stage one, and Ostberg had a spin on stage three, and coped with running unsuitable hard tyres throughout the loop.
Citroen managed to salvage third and fourth from the morning, and by stage three its works cars were setting the pace again.
Mikko Hirvonen and Sebastien Loeb are currently 13s and 19s respectively behind Ostberg and poised for a comeback charge.
But the factory Ford team's hopes of winning its final rally in its current incarnation look remote already.
Jari-Matti Latvala led after stage one, then struggled a lot with the slippery conditions and took things too carefully on stage three.
He is now 55s off the lead, but still in better shape than team-mate Petter Solberg, who hit a rock on stage two and had to retire.
Ironically the same obstacle was Solberg's undoing in Spain in 2011 - and this year it also caught out Thierry Neuville and Andreas Mikkelsen.
Another big name retirement was Dani Sordo, who put his Prodrive MINI off the road on stage three.
But his new team-mate Jarkko Nikara is thriving. The former Pirelli Star Driver is 14s behind Latvala in sixth, and holding Sardinia runner-up Evgeny Novikov at bay.
Chris Atkinson has had a troubled morning in eighth, with Citroen newcomer Hans Weijs ninth and SWRC leader Craig Breen 10th. Breen's title rival PG Andersson led the class until losing a minute with a differential issue.
Yeray Lemes is just 2s ahead of Marcos Ligato in a tight PWRC battle.