Mikkelsen remained 0.5 seconds clear of Rovanpera after the day's first stage, but the Finn overhauled him in a dramatic three-way battle including Britain's Elfyn Evans.
Mikkelsen won the day's opening stage but dropped to third behind Evans after two stages.
Rovanpera won six of the day's seven stages to overturn the 1.8 sec advantage held by Mikkelsen on Friday night.
Four speed tests on Sunday stand between Rovanpera and a remarkable win.
Two-time defending world champion, the Finn was not even due to be racing in Poland but had to replace Sebastien Ogier at short notice after an accident during a reconnaissance run on Tuesday.
The two-time Rally Poland winner and an eight-time world champion, Ogier and his co-driver Vincent Landais, had to withdraw from the event after the crash.
The world's top drivers are battling for honours in the 80th annual Rally Poland, the seventh round of the WRC season.
In all, 42 crews from 23 countries are taking part, including some from as far afield as Bolivia, Argentina and Peru.
The rally opened on Thursday in the northeastern lakeside resort of Mikołajki and runs until Sunday.
(mo)
Source: Reuters, AFP