Hyundai's Lappi heading for first victory since 2017 at Rally Sweden
The Finn's chances were boosted when Toyota's Takamoto Katsuta, his closest rival overnight, crashed into a snowbank and got stuck on Saturday's second stage after closing the gap to under a second.
Under the new scoring system for 2024, Lappi is guaranteed 18 points if he negotiates the snowbanks and other hazards and reaches the finish after three more stages on Sunday.
He played safe on Saturday afternoon, preserving his tyre studs and finishing one minute 06.3 seconds ahead of M-Sport Ford's Adrien Fourmaux, with Toyota's Elfyn Evans third.
"We were trying to manage the lead and not take any risks in a bid to save the tyres for the final stages," said Lappi, whose only win to date was in Finland.
"There are still three stages to go, and two long stages in particular where anything can happen. I am happy we are in this situation but it isn’t over yet."
Hyundai's championship leader Thierry Neuville, winner of the Monte Carlo opener last month, was in fourth place and two minutes 22 seconds off the lead on a day with five different winners in seven stages.
Twelve points can still be won on Sunday, five of them in the closing Power Stage.