Heavy rain and standing water made the asphalt circuit and tricky interior dirt roads incredibly slippery. Having set second fastest time on the day’s opening 13.43km Scorpion speed test, the Finn gambled by selecting heavily treaded snow tyres for the re-run.
The extra grip enabled Lappi to win the stage by 7.9sec in his Ford Fiesta and vault into the lead of this final round of the FIA World Rally Championship season.
“A bit risky! We need to see how much they are wearing but it seems like it was working,” smiled Lappi afterwards.
Video: Ogier without vision
He was 3.3sec clear of Dani Sordo, who won the first stage to demote overnight leader Sébastien Ogier. Sordo had a scare when the engine of his Hyundai i20 briefly stopped and would not restart after SS3 before eventually firing into life.
Andreas Mikkelsen held a remarkable third, 22.2sec off the lead. The Norwegian matched the more powerful World Rally Cars in his WRC 3-specification Skoda Fabia Evo as the additional power brought no benefit to the bigger cars in the slippery conditions.
Ogier was fourth in his Toyota Yaris, and first of the four men chasing the drivers’ world title. The Frenchman was just 0.2sec adrift of Mikkelsen, despite briefly stopping after clipping a bale in the opener and struggling for visibility as his windscreen misted up during SS3.
Ott Tänak was fifth, the Estonian receiving a shock when the driver’s door of his i20 flew open several times during SS2. He was only 1.1sec behind Ogier and 2.9sec ahead of championship leader Elfyn Evans.
Evans briefly stalled his Yaris’ engine in the opener, but the Welshman remains on course to secure his maiden title. Team-mate Kalle Rovanperä was seventh ahead of WRC 2 leader Adrien Fourmaux, who shone in his Fiesta.
Thierry Neuville was ninth after a troubled morning. The Belgian slid into a fence in SS2 and dropped almost 30sec as he battled to restart his i20’s engine. He plunged to 13th, but third fastest on the re-run enabled him to break back onto the leaderboard.
“After the surprise this morning when we were caught out by the mud, we have to push hard. I tried to do good, but the conditions are very bad and it’s easy to do a mistake,” he said.
Ole Christian Veiby completed the top 10, the Norwegian steering clear of problems on his World Rally Car debut in an i20.
Teemu Suninen limped through both stages with an engine problem in his Ford Fiesta. “I have maybe two or three cylinders and no power at all,” said the M-Sport Ford driver, who was 13th.
Team-mate Gus Greensmith retired when he hit a gate and broke his front right suspension.
Full coverage from ACI Rally Monza is available on WRC+ All Live here, including every stage broadcast as it happens as well as key interviews, features and expert analysis from the service park.