Eight-time world champion Ogier took a 16.3s lead over Lappi heading into the afternoon's three stages after winning two of the three morning tests, held in largely dry conditions. However, Lappi starred in the wet to grab the rally lead on the final stage of the day, despite picking up a left rear puncture.
Lappi's team-mate Thierry Neuville headed to the service in third, 18.6s adrift after a handbrake issue, while world champion Kalle Rovanpera climbed from seventh to fourth but remains 46.1s in arrears.
Toyota’s Takamoto Katsuta (+47.4s), Elfyn Evans (+1m05.6s) and M-Sport’s Ott Tanak (+1m09.8s) completed the top seven. M-Sport's Pierre-Louis Loubet crashed in the day's final stage, while Dani Sordo lost more than three minutes to a roll in the morning in the third Hyundai.
The weather closed in for the start of the afternoon loop as spots of rain began to fall at the start of stage five (Tantariles), the roughest of the morning loop.
Ogier was unable to repeat his stage-winning performance of the morning as nearest rival Lappi came to the fore, slashing the Frenchman’s rally lead from 16.3s to 8.7s.
The Finn produced a mistake free run and was aided by a decision to take four softs and one hard tyre. Ogier, who was 7.6s slower, carried an extra spare wheel having selected two hards and four softs.
There was however drama for third-placed Loubet as in-car cameras captured the Frenchman desperately trying to engage a gear, before moving to the start line. The issue stopped him from starting the stage in his correct time slot, resulting in a three-minute penalty.
Loubet eventually passed through the stage in road mode after officials were forced to cancel the stage due for spectator safety reasons. Seven Rally1 entries had already passed through at this point, Sordo the only other Rally1 car to run through at road speed and also handed a nominal time.
Neuville, who mirrored Lappi’s tyre strategy, clocked the second fastest time prior to the stoppage which helped the Belgian climb to fourth overall. He set the time despite battling intermittent handbrake issues.
Evans was fourth fastest ahead of Tanak, who found an improved balance in his M-Sport Ford Puma.
Hyundai’s strong start to the afternoon continued when Neuville claimed stage six (Terranova), despite ongoing handbrake woes.
Neuville was 2.6s faster than Lappi, who in turn took another two seconds out of Ogier’s lead in a stage that had become considerably rutted, especially in the treacherous wet and muddy final sector.
Evans appeared to struggle the most with this sector, the Welshman proving the slowest Rally1 runner with a time 9.1s shy of Neuville that dropped him to fourth overall.
“It's a bit crazy now, you need a four-wheel drive pickup in there I think. It's like Dakar," said Evans.
The stage was red flagged after the Rally1 crews passed through when WRC2 title contender Gus Greensmith went off the road. Greensmith and co-driver Jonas Andersson were okay, although the former has been taken to hospital for precautionary checks due to the significance of the impact.
UPDATE: Greensmith will remain in hospital overnight for observation.
The rain intensified for the second pass of the 49.9km Monte Lerno test. This played into the hands of Rovanpera, who was first into the test.
The Toyota driver, renowned for his skills in wet and slippery conditions, pushed hard to score a first stage win of the event. He jumped from seventh to fourth overall after posting a time 2.1s faster than Lappi.
Lappi impressively managed a left rear puncture to take 6.8s out of Ogier through the stage to snatch the rally lead.
"There was no need to push harder, one tenth is enough,” joked Lappi. “Glad to be at the finish, that was quite a tricky stage and we got a left rear puncture as well."
There was however further drama for M-Sport as Tanak suffered water pump issue, which cost him 32.9s. The day worsened for team-mate Loubet, who appeared to suffer a mechanical issue which led to him crashing out during the stage.
Evans also hit trouble when he suffered a front right puncture with seven kilometres remaining in the stage resulting in a 42.7s time loss.
Toksport Skoda's Sami Pajari led M-Sport's Adrien Fourmaux in WRC2 ahead of Saturday's eight stages.