World rally champion Kalle Rovanpera snatched the Rally Finland lead from Toyota team-mate Elfyn Evans at the end of an eventful Friday morning that included a crash for Ott Tanak.
Rovanpera headed to midday service with a 0.2s margin over Evans, who had impressed in difficult wet conditions in his bid for a first win of 2024.
Toyota's Sebastien Ogier briefly held the rally lead after stage two, but returned to service 3.0s adrift in third.
Hyundai's Esapekka Lappi led Hyundai's charge in fourth [+9.7s] ahead of championship leader Thierry Neuville [+14.1s], with M-Sport Ford's Adrien Fourmaux sixth [+31.3s] and team-mate Gregoire Munster in seventh [+1m08.3s].
Overnight rain continued into the morning, resulting in slippery conditions for the crews. The weather favoured those at the top of the road order, although the wet gravel proved particularly treacherous.
Starting second on the road, Evans managed to tame the conditions in stage two [Laukaa, 17.96km] to take the stage win by 0.4s from Ogier despite admitting it was "difficult to judge the grip".
But it was Ogier who took the rally lead by one second from fellow part-time entrant Rovanpera, who was third fastest.
"It's really slippy and quite a lot of mud from the cuts, today it is quite good to be in front," said defending WRC champion Rovanpera.
Team-mate Takamoto Katsuta made it a Toyota 1-2-3-4 on the timesheets despite being clearly frustrated with his driving, which he described as "pretty shit".
The fifth Toyota fared even worse as Rally1 debutant Sami Pajari suffered a wild spin before understeering into a ditch which damaged the rear wing and rear of the car, but he was able to reach the finish after dropping 17.8s.
"Extremely tricky," remarked Pajari. "I don't know where I am. We can keep going, it's fine."
Overnight leader Neuville led Hyundai's charge, albeit four seconds adrift after a run he labelled a "disaster". His effort was still 0.6s faster than team-mate Lappi, while Tanak was 0.9s further back ahead of a cautious Fourmaux.
The Frenchman's team-mate Munster was the last of the Rally1 runners, after losing time to a stall.
The all-new stage three [Saarikas 15.93km] provided a potentially pivotal moment in the championship when Tanak was involved in a scary high-speed crash that red-flagged the test for 19 minutes.
The three-time Rally Finland winner lost control of his i20 N mid-corner, which pitched the car into a roll before colliding with a tree. Tanak and co-driver Martin Jarveoja exited the car, before the latter was taken to hospital for precautionary checks.
Once action resumed, Rovanpera clocked the fastest time on the slick gravel, which was enough to snatch the overall rally lead.
He was 0.4s quicker than Evans, with Lappi third ensuring three cars were split by less than a second.
Previous rally leader Ogier admitted he was "missing some commitment" and subsequently dropped two seconds, which dropped him to third overall behind Evans.
Neuville's difficult start to the day continued when he misjudged a junction, resulting in his i20 N running straight onto the grass and costing him 12 seconds. The Belgian was the only car through the test before the stoppage caused by Tanak.
The rally lead changed hands for a third time after stage four [Myhinpaa, 15.51km], as two-time Finland winner Evans shot to the summit of the leaderboard.
Those at the top of the order benefitted from changeable weather as Neuville, Evans and Ogier avoided a localised heavy rain shower that affected their rivals.
Evans took the stage win by 0.8s from Neuville, which was enough to move Evans into a 1.2s rally lead over Rovanpera.
Rovanpera was among the first to meet the rain in the middle of the stage and reached the end only 0.8s slower than Ogier, but the world champion was frustrated with the performance of his GR Yaris.
"I am fighting a lot with the car and I'm doing some set-up changes for every stage, but this one was f****** horrible," said Rovanpera. "The car is oversteering so much that on the fast road, you cannot be fast. I cannot do much more."
Katsuta declared parts of the stage as "impossible" due to the conditions while Fourmaux, struggling for speed having started the event without a pre-event test, felt his run was a "disaster" as the Ford driver sat in seventh overall.
The remaining Rally1 runners made it through unscathed, with Pajari the slowest as he battled with his GR Yaris's damaged rear end.
The morning's final stage [Ruuhimaki 7.76km] produced another twist as Katsuta surrendered fourth when he clipped a tree.
The impact damaged his right rear suspension and ripped his wheel off, although the Japanese attempted to bring the car back to service.
At the front, Rovanpera won the stage by 0.6s to snatch the rally lead back from Evans.
The crews will repeat the loop and run through the Harju stage to complete Friday's action.
