The Finn, making only his fourth World Rally Car start, headed Toyota Gazoo Racing team-mate and fellow countryman Jari-Matti Latvala by 4.4sec after winning eight of today’s 12 speed tests.
Home drivers bossed the leaderboard with fellow novice Teemu Suninen third in a Ford Fiesta, a further 14.6sec behind.
It was a disappointing day for title contenders Sébastien Ogier and Thierry Neuville. Championship leader Ogier crashed this morning, while Neuville struggled for pace in eighth.
A cautious Lappi trailed 12th after this morning’s opening dirt road special stage but reeled off eight wins from nine special stages to overhaul Latvala and surprise himself.
“I couldn’t even imagine leading,” said the 26-year-old Toyota Yaris pilot. “It’s really something special and difficult to understand. Ever since I was a young boy I’ve dreamed of driving a World Rally Car in Finland. Now it’s happening and I’m leading. I’m speechless.”
Latvala led after Ott Tänak yielded the early initiative when he swiped some rocks and punctured his Fiesta’s right rear tyre. The Estonian lost more than 90sec to end 10th.
Suninen’s performance was almost as impressive as Lappi’s. In only his second drive in a World Rally Car, the 23-year-old won two tests during an epic 16-hour day in sunshine and heavy rain.
Craig Breen, Juho Hänninen and Mads Østberg joined the battle for a podium place. Brake problems and handling issues delayed Breen’s Citroën C3 and the Irishman was fourth, 14.0sec behind Suninen.
Hänninen provided more Finnish delight when he held fourth before hitting a rock and damaging the rear left suspension mounting on his Yaris. He was lucky to survive in fifth, 6.1sec behind Breen and 5.9sec ahead of Østberg.
The Norwegian fell back from third when handling issues, blamed on either a damaged track control arm or broken driveshaft, relegated him to sixth.
Ogier damaged his Fiesta’s rear suspension after a heavy landing over one of Finland’s many jumps and shortly after he crashed into trees when it failed altogether.
His demise handed the initiative to Neuville in their title duel. But the Belgian was subdued and faces a massive battle to achieve his target of a top five finish. “Tomorrow is a huge day. We should be faster than this, but I couldn’t do more,” said the Hyundai i20 driver.
Today’s other major retirement was Hayden Paddon, the Kiwi breaking his i20’s suspension after clattering a rock.
Saturday packs in eight more gravel stages near Jämsä. They are split into two identical loops and include the classic rollercoaster Ouninpohja, regarded as the sport’s biggest test of bravery and commitment. It all adds up to 132.34km of action.
1. E Lappi / J Ferm FIN Toyota Yaris 1hr 11min 36.4sec
2. J-M Latvala / M Anttila FIN Toyota Yaris + 4.4sec
3. T Suninen / M Markkula FIN Ford Fiesta + 19.0sec
4. C Breen / S Martin IRL Citroën C3 + 33.0sec
5. J Hänninen / K Lindström FIN Toyota Yaris + 39.1sec
6. M Østberg / T Eriksen NOR Ford Fiesta + 45.0sec