After 15 special stages, the flying Finn's Toyota was 9.4 seconds ahead of Norway's Andreas Mikkelsen (Hyundai) with Welshman Elfyn Evans, his Toyota team-mate, back in third at 16.1 seconds.
Rovanpera won six of the day's seven stages to overturn the 1.8sec advantage held by Mikkelsen on Friday night.
"I tried my best the whole day, and the afternoon was definitely more enjoyable when I knew what was coming," Rovanpera said.
Four speed tests on Sunday stand between Rovanpera and a remarkable win.
The Finn was not scheduled to drive in Poland but had to replace Sebastien Ogier at short notice after an accident during a reconnaissance run on Tuesday.
The eight-time world champion and his co-driver Vincent Landais, who was involved in a head-on collision, were uninjured, as were the two occupants of the other vehicle involved.
"It will not be easy tomorrow. I think it will be tougher than today. Let's hope for the best," said Rovanpera.
Mikkelsen won the day's opening stage but dropped to third behind Evans after two stages.
But the Welshman suffered a puncture on SS13 which cost him almost 10 seconds and leaves him trailing Mikkelsen by 6.7sec going into the final day.
Evans at least picked up 13 points during the day which meant he outscored championship rivals Thierry Neuville (6) and Ott Tanak (0) with the latter, who had already stopped on Friday, retiring his Hyundai again this lunchtime in a bid to preserve the car for Super Sunday.
Standings after 15 special stages:
1. Kalle Rovanpera/Jonne Halttunen (FIN/Toyota) 2hrs 0min 44sec, 2. Andreas Mikkelsen/Torstein Eriksen (NOR/Hyundai) at 9.4secs, 3. Elfyn Evans/Scott Martin (GBR/Toyota) 16.1, 4. Adrien Fourmaux/Alexandre Coria (FRA/M-Sport Ford) 37.0, 5. Martins Sesks/Renars Francis (LVA/Ford) 58.2
