The Finn successfully navigated the 3.23km test, held across a mix of tarmac and gravel surfaces in host city Olbia, to clock a time 0.2s faster than M-Sport’s Ott Tanak.
"This is fun. You have enough space, it's wide enough, you can play with the car,” said Lappi, who is planning to run a measured gamble on set-up for this weekend’s rally.
“On the gravel you just go fully sideways and keep the foot on the throttle because it's so slippery, that's the only way to do it.”
Tanak produced a committed run through the test to reach the finish just shy of the Finn’s earlier benchmark. The 2019 world champion is running upgraded suspension components on his Ford Puma, which he feels has improved the car.
"We did a good job in between [rallies], it was necessary after Portugal,” said Tanak, who encountered a small scare going over one of the jumps in the stage.
“I wouldn't say we should be too confident at the moment but for sure we are better than in Portugal.”
Hyundai’s Thierry Neuville was third only 0.5s adrift of team-mate Lappi, while Takamoto Katsuta was the fastest of a fleet of four Toyotas, a further four tenths behind Neuville.
Reigning world champion and championship leader Kalle Rovanpera, who will start Friday's stage first on the road, was fifth quickest a tenth faster than team-mate and four-time Sardinia winner Sebastien Ogier.
Hyundai’s Dani Sordo (+1.8s), Toyota’s Elfyn Evans (+2.0s) and the sister M-Sport entry driven by Pierre-Louis Loubet (+2.1s) rounded out the Rally1 field.
Oliver Solberg topped the leaderboard in the WRC2 class after posting a time, 0.2s faster than fellow Skoda driver Miko Marczyk.
Crews will faces six rough gravel stages on Friday including the new extended 49.9km Monte Lerno test.
Drivers are expecting a gruelling day after likening the stages to those driven at Safari Rally Kenya following this week’s recce.
