The Estonian ended the day 25.9 seconds clear of Spaniard Dani Sordo, and looks set to wrest the championship lead from Sebastien Ogier on Sunday.
"It will be difficult with Ott but I’ll keep trying," said Hyundai's Sordo.
Ogier, who is out of contention for the rally win after hitting a rock and retiring his Citroen on Friday's opening leg, returned to the fray but hit another boulder and sustained more suspension damage.
The Frenchman was able to make repairs this time and continued.
Ogier was two points clear of Tanak before Sardinia, with Hyundai's Thierry Neuville a further eight points adrift.
Sunday features four more stages with a final 'power stage' near Sassari. (Reporting by Alan Baldwin in London, editing by Pritha Sarkar)