The reigning World Rally champion was 1.3 seconds quicker than Volkswagen team-mate Jari-Matti Latvala on the short opening Lisboa stage.
VW filled the top three places, with Andreas Mikkelsen one of the early pacesetters and slotting into third place.
Hyundai took best of the rest honours as Thierry Neuville put the i20 fourth, ahead of Citroen's Kris Meeke and M-Sport's leading driver Mikko Hirvonen
Tanak eyes star showing in Portugal
The full-length stages commence on Friday morning.
Heavy rain disrupts WRC Portugal recce
Latvala leads Portugal shakedown