Rovanpera managed to tame changeable conditions to win three of the four high-speed gravel stages to lead Toyota team-mate Elfyn Evans by 3.9s at midday service.
Hyundai’s Thierry Neuville completed the loop in third (+12.3s) while Toyota’s Takamoto Katsuta inherited fourth (+16.0s) after Esapekka Lappi crashed out in stage five.
Teemu Suninen (Hyundai) and WRC returnee Jari-Matti Latvala completed a reduced Rally1 field after M-Sport duo Ott Tanak and Pierre-Louis Loubet exited the rally in stage three. Oliver Solberg headed into Friday afternoon in seventh as the fastest of the WRC2 runners.
Overnight rain created damp road conditions which played into the hands of Rovanpera, faced with the task of opening the roads as the points leader.
The Finn managed to usurp overnight rally leader Tanak by setting the third fastest stage time on stage two, as Toyota dominated the timing screens.
"Quite slippy conditions, the grip is not so good, so definitely not so easy,” said Rovanpera, who was 1.3s adrift of the ultimate pace. Team-mate Katsuta took the stage win, making the most of his road position to pip Evans.
Lappi was the quickest of the closely-matched Hyundai trio as 0.6s covered the 2017 Finland winner, Neuville and Suninen. After winning Thursday’s opening stage, Tanak was a tenth slower than Suninen although the Estonian wasn’t entirely comfortable.
Moments later disaster struck Tanak and the M-Sport team as its pair of Ford Pumas retired from the day’s action on stage three. The 2019 world champion suffered a loss of power at the six kilometre mark during the famous high-speed Lankamaa test, which has returned to the rally in this format for the first time since 1993.
Source: Autosport