Tanak chalked up three stage wins to end the day 3.8 seconds clear of Finland's Esapekka Lappi, the 2017 winner who was quickest on the day's last three stages.
Last year's winner Britain's Elfyn Evans was 19.3 seconds off the pace in third, with championship-leading Finnish team mate Kalle Rovanpera fourth, after having the disadvantage of running first as 'road sweeper', and a further 1.7 seconds behind.
"We definitely did not expect to be in the fight, so to end the day in this position is really positive," said Tanak.
"The other boys were quite slow to wake up, but it seems the Toyotas are slowly getting to where they should be."
Saturday features eight stages and is the longest day of the event, with 150.3km of competitive action on the fast gravel roads in the forests around Jyvaskyla.
"With the starting place we will have tomorrow it should be a bit easier to fight, and let’s see what the weather brings," said Rovanpera.
The 21-year-old Finn leads Hyundai's Belgian Thierry Neuville, seventh on Friday at Rally Finland, by 83 points in the championship after seven of 13 events.