Kalle Rovanpera groped through the fog and grabbed the Chile Rally lead from Toyota teammate Elfyn Evans as the mist rolled off the Pacific late on Sat
"They were really difficult conditions," said Rovanpera. "Huge fog, and some of the most challenging stages of the year, I think. I have never done anything like this, it's crazy. You drive and you are just trying to stay on the road."
Reigning champion Rovanpera is only racing part time this year and is not in contention for the overall title.
Evans is a distant fourth in the standings with two rallies to come after Chile.
French driver Sebastian Ogier, who is third in the standings, was forced out on Saturday.
Ott Tanak, who is second in the standings, finished Saturday third on the road.
Going into Sunday's final four stages, Finn Rovanpera holds a 15.1sec lead over Welshman Evans, Estonian Tanak is another 18.5sec back in third ahead of Hyundai teammate Thierry Neuville, the championship leader.
But the Belgium also made the most of the fog cutting more than 20 seconds out of Tanak's advantage on the last two legs.
Evans had also been second behind Tanak at the end of Friday's racing but was promoted to first overnight after marshalls revised the Welshman's "notional" time for a stage he did not complete.
Friday's opening stage was abandoned with some drivers still on the course because fans were standing in dangerous spots.
After the race covered the same section in its afternoon programme, marshalls decided to credit drivers who did not finish in the morning with the time they posted in the afternoon.
That gave Evans a three-second lead. He increased it steadily as he won three of the four morning stages on Saturday.
But when the fog thickened for the last two stages of the day, Evans, the last driver on the road as leader, struggled.
"I couldn't see beyond the bonnet, it's crazy," said Evans after the penultimate stage.
Ogier's hopes were dashed at the end of the eighth stage when he hit a stone on the side of the road. The impact broke the front suspension of the Frenchman's Toyota, which crashed into the embankment a little further on, forcing the eight-time world champion to retire for the day.
If he starts again on Sunday, he can still score a few points in the last special stage, the Power Stage.