Lappi, like Rovanpera, is competing only part-time this season with Tanak and Belgian Thierry Neuville, winner of the Monte Carlo opener, the main title contenders for the manufacturer.
The Finn used road position advantage in heavy snow to end the day 3.2 seconds clear of Toyota's Japanese Takamoto Katsuta with Sweden's WRC2 leader Oliver Solberg a distant third in his Skoda.
"This has been one of the toughest snow afternoons for me ever," said Lappi, who won four of the eight stages.
"I have never driven in conditions like this, it’s pretty extreme even for a Finnish driver. So far we have been fastest, so I am hoping for even more snow."
Championship leader Neuville had been first on the road in the morning in poor visibility and then suffered mechanical problems in the afternoon when he was four minutes late to a stage and collected a 40-second penalty.
The Belgian ended the day in 11th place.
Tanak hit a snowbank and spun on stage four but was able to get back to the service area and will rejoin the rally on Saturday, as will Rovanpera who had to stop with radiator damage after also making contact with a snowbank.
"We were having a good start to the rally and a good chance to fight for the win," said Rovanpera, also making his first start of the season.
"We just went a bit wide in a right-hand corner. I think some drivers in front had already touched the bank so there was a big hole in it which just kicked us into a spin.
"We damaged the radiator and also picked up a puncture in the same moment, and although we could make some repairs we unfortunately could not continue today."
