Craig Breen could not retain his overnight fourth at the penultimate leg of the Rally Finland after struggling for rhythm in his Citroen C3 and surviving a spin and an overshoot.
The Waterford man was 14.7 seconds clear of title challenger Thierry Neuville (sixth), who hopes to capitalise on the absence of championship leader Sebastien Ogier.
Breen slipped to fifth overall, with Northern Ireland's Kris Meeke eighth.
Finland's Esapekka Lappi is closing in on a first World Rally Championship victory in his home country.
Lappi, starting only his fourth event at the top level, ended the day with a 49.1 seconds lead in his Toyota Yaris over fellow novice Teemu Suninen.
Only Sunday's short final leg separates Lappi from becoming the sixth different winner in nine WRC rounds.
Lappi's Toyota Gazoo Racing team-mate Jari-Matti Latvala had appeared well placed after winning the first five of Saturday's eight dirt road speed tests.
Latvala overturned Lappi's overnight advantage to build an 8.5secs lead over his Finnish colleague.
But his hopes of a fourth victory in his home event ended when his Yaris ground to a halt with a suspected electrical problem, leaving Lappi clear.
"I'm disappointed for Jari-Matti but it's not my fault his car broke down so I need to enjoy this moment now," Lappi told www.wrc.com.
"It's something amazing. "We led (on Friday) by four seconds but this is 50 seconds and I really need to concentrate to keep my head cool."
Juho Hanninen broke his Yaris' front anti-roll bar to trailed Suninen by 4.3secs in an all-Finnish top three.
Welshman Elfyn Evans was only 1.3 seconds behind in fourth after an impressive drive in his Fiesta which culminated in a final stage win.
1 Esapekka Lappi (Finland) Toyota 2:13:02.700 2 Teemu Suninen (Finland) Ford +00:49.100 3 Juho Haenninen (Finland) Toyota 00:53.400 4 Elfyn Evans (Britain) Ford 00:54.700 5 Craig Breen (Ireland) Citroen 01:16.900 6 Thierry Neuville (Belgium) Hyundai 01:31.600 7 Ott Taenak (Estonia) Ford 02:07.500 8 Kris Meeke (Britain) Citroen 02:59.100 9 Daniel Sordo (Spain) Hyundai 03:54.000 10 Mads Ostberg (Norway) Ford 04:23.600