The current leader of the world championship, Kalle Rovanperä, was only 4th, some 21 seconds behind Tänak when he was expected closer to the lead for his national rally.
The Estonian, WRC world champion in 2019 and already double winner in Finland (2018, 2019), won three special stages out of the eight contested on Friday.
One, the 5th, was canceled for security reasons.
"I'm pretty flat and can't wait to get to bed," said Tänak at the finish of the day's final stage, expressing concern about the pressure Lappi was putting on.
The latter, who won this event in 2017, was only 3.8 seconds behind Tänak at the end of this second day of the rally which is due to conclude on Sunday.
"I'm driving very well at the moment, very cleanly and if I pushed harder I wouldn't necessarily go faster," he said after his victory in the 8th special.
He won a total of four on Friday, one more than Tänak.
The 31-year-old driver has not raced regularly in the WRC since 2020 but had signed a 4th place last year here in Finland, already on a Toyota.
He was invited again this year by the Japanese team to make up for the absence of Sébastien Ogier, the eight-time French world champion who only takes part in a few selected events.
Tänak and Lappi widened the gap on the 3rd, the Briton Elfyn Evans (Toyota), who found himself Thursday evening 19.3 seconds behind the leader with Rovanperä just behind him.
Belgian Thierry Neuville (Hyundai), current 2nd in the world championship, did not seem to be on the pace and was only in 7th position 50.2 seconds behind Tänak, ten seconds ahead of Frenchman Pierre-Louis Loubet (M- Ford Sports).
Adrien Fourmeaux, the other Frenchman hired by M-Sport Ford in Finland, experienced mechanical problems on Friday morning which caused him to dive in the standings.
The Swede Oliver Solberg (Hyundai) violently left the track in the second special stage and was unable to restart, his car being too damaged.
Justus Räikkönen, the nephew of ex-F1 world champion Kimi Räikkönen, entered in WRC4 for his first world championship rally at the age of 17, was at the end of the day in a meritorious 30th place, first in his category.
© 2022 AFP