The Toyota driver, only 13 points behind Hyundai's championship leader Thierry Neuville with two rounds remaining after Britain, was 28.8 seconds ahead of the Belgian after nine stages.
Tanak arrived in Wales fresh from victories in Finland, Germany and Turkey that have thrust him into title contention ahead of M-Sport Ford's defending world champion Sebastien Ogier.
"It was a really good day," he said after a day of five stage wins. "It looks like we’ve made a good step with the car and in the really low-grip conditions we were competitive."
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Frenchman Ogier, the five times world champion who is joining Citroen next year, was running in fifth place and 38.2 seconds off the lead after losing first and second gears and also spinning.
Ogier changed the gearbox and clutch in the service park before the afternoon stages.
(Reporting by Alan Baldwin, editing by Toby Davis)
