A controlled run through the Saturday afternoon stages meant he extended his advantage to 39.9s despite a minor power steering worry.
Tanak's escape was helped by erstwhile lead rival Mads Ostberg struggling with the tyre wear on his Citroen, as he had towards the end of Friday's afternoon loop.
Even though Ostberg took two spare tyres compared to most others' one, he still felt he had run out of rubber by the start of the day's final stage.
Evans takes world championship lead with Sweden win
That encouraged Jari-Matti Latvala to push to give Toyota a one-two, and he closed the gap to Ostberg from 20s to 5.4s by the end of the leg.
Toyota now has three cars in the top four, as last year's Finland winner Esapekka Lappi continued his charge and grabbed fourth from lead Hyundai runner Hayden Paddon.
Lappi, who had fallen to 10th when his car stalled on Friday's opener, was fastest on three of the afternoon's four stages.
Behind Paddon, M-Sport Ford still holds sixth, seventh and eighth with Teemu Suninen, Sebastien Ogier and Elfyn Evans.
Ott Tanak - Rallye de Finlande 2018
Image credit: Getty Images
WRC title contender Ogier has not been able to make up any ground during Saturday and said M-Sport's new aero package - fitted only to his car - is simply not working yet, lamenting the fact he had only one day of testing with it.
But Ogier is still on course to outscore championship leader Thierry Neuville, who is a distant 10th behind Craig Breen.
The third Hyundai of Andreas Mikkelsen is back up to 11th after crashing on Friday but is four and a half minutes away from the points.
Kalle Rovanpera had dominated WRC2 for Skoda until the day's penultimate stage when he hit a rock and sustained suspension damage.
Initially fearing he would be out, Rovanpera managed to effect a makeshift repair so he could limp to the end of the day - which he managed despite having to drive most of the final stage with the Fabia's bonnet obscuring his windscreen after it flew up.
Eerik Pietarinen has now picked up the WRC2 lead in a Skoda run by ex-WRC driver Toni Gardemeister's TGS Worldwide team.
Leading positions after SS19
Tanak wins in Chile, Ogier retakes championship lead
World championship leader Neuville crashes out in Chile
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