A half-spin in the opening dirt road speed test left him trailing early leader Sébastien Ogier by nearly 25sec in his Toyota Yaris.
Tänak stormed back up the leaderboard, winning five of the next six special stages to build a 22.7sec advantage over Kris Meeke and leave his rivals open-mouthed.
“It was a stupid mistake. I hit a rock, had a spin and broke something in the steering which caused problems in the next stages. For the moment we’re performing quite well and this car starts to suit me,” said the Estonian, in his first season with Toyota Gazoo Racing.
His dominance was emphasised by the fact the next five drivers were covered by less than 20sec. A final stage puncture cost Meeke a few seconds but the Citroën C3 driver admitted it made no difference. “It was impossible to follow Ott, he’s been on a blinder,” he said.
Thierry Neuville bounced up and down the order all day. The Hyundai i20 driver eventually snatched third in the final test from team-mate Dani Sordo, having gambled on hard compound tyres for the afternoon tests as temperatures climbed to 30°C.
They were split by 0.9sec, with championship leader Ogier a further 6.9sec behind in a Ford Fiesta after a sterling job limiting his time loss from first in the start order.
Ogier swept loose stones from the roads to leave a cleaner and faster line for those behind. His pain was eased initially as hanging dust slowed those behind, forcing organisers to extend the gap between cars from two to three minutes.
Craig Breen has little experience of the bumpy Argentine roads but was sixth in a C3. His lack of knowledge meant he struggled with his pace notes and he clipped roadside banks on several occasions.
Jari-Matti Latvala was the only major retirement after hitting a rock and breaking his Yaris’ front right suspension when third. The damage also caused his engine’s oil system to fail and he will not restart