Lappi made the most of an advantageous road position on the rough gravel stages to win two of the three tests that made up the morning loop.
The Finn headed to the midday service with a 1.4s lead over Ogier, who won Stage 4 after starting the day a position ahead of Lappi on the road order.
The pair surged to the head of the field after overnight leader Ott Tanak suffered turbo failure on the day’s opening El Chocolate stage, which resulted in the Estonian dropping outside of the top 20 after losing more than 12 minutes limping through the three stages.
Toyota’s Elfyn Evans successfully navigated the stages to end the loop in third, 12.5s adrift and 0.6s ahead of Hyundai’s Dani Sordo (+13.1s) with Thierry Neuville (+23.2s) fifth and Kalle Rovanpera (+29.2s) sixth.
M-Sport duo Pierre-Louis Loubet and Jourdan Serderdis retired on Stage 3 while Toyota’s Takamoto Katsuta crashed out on the morning's final test.
The day started in a disastrous fashion for M-Sport as all three Ford Pumas hit trouble on the infamous 29.07km El Chocolate stage that opened the day.
Road sweeper Tanak already faced the disadvantage of sweeping the road before his car then suddenly lost power 15.8km into the test due to a turbo failure.
The Estonian pulled over to attempt to fix the problem but lost more than seven minutes in the process and was forced to let Rovanpera and Neuville through.
Moments later his team-mates Loubet and Serderidis ground to a halt. Loubet, contesting a first rally outside Europe, clouted a rock with the front right at tight right hander that put him out on the spot.
Serderidis somehow lost control of his Puma and was left facing in the wrong direction which forced officials to red flag the stage.
The stoppage came with only eight Rally1 cars completing the test, the fastest was Lappi, who managed to pip Ogier by 0.6s to move into the rally lead.
Sordo was third fastest on the stage ahead of Evans, Neuville and Rovanpera, the latter became the effective road sweeper after Tanak’s turbo problem.
Katsuta was fortunate to complete the test after complaining about smoke coming from inside his GR Yaris that hampered his visibility.
Ogier fought back on Stage 4 to claim his first fastest time of the event after pipping Lappi by 0.4s.
The pair were able to avoid a bump that almost caught out Rovanpera and Neuville. The pair were surprised by a compression that hadn’t been there when they passed through the stage in the recce earlier in the week.
“After the recce somebody made a new speed bump on the stage. We came through at speed and the hit was quite big,” said Rovanpera.
Neuville added: “It was crazy and very very painful. A bump came as a surprise and I thought we were going to go off. It's in a really fast section but luckily we are here and the car is working more or less.”
The pair would later suffer a loss of hybrid in the following stage which Neuville attributed to the impact with the bump. It was later confirmed by WRC that the bump in stage 4 was removed by officials.
Lappi responded to Ogier’s attack on the morning’s final stage to again pip the six-time Rally Mexico winner by 0.6s to lead going into the afternoon.
The Las Minas stage wasn’t without drama as Katsuta’s hopes of championship points were dented after running off the road. The Japanese, who was sitting seventh, lost control of the rear of his GR Yaris and slid off the road and down a steep incline.
Luckily with he and co-driver Aaron Johnston were okay but it spelled a premature end to their day.
In WRC2, reigning champion Emil Lindholm reached service with a 3.0s lead over Gus Greensmith, with Adrien Fourmaux, third and Oliver Solberg, fourth.
The crews will repeat the three stages that made up the morning loop before tackling two superspecial tests to end the day.
