Competing in his third event for Hyundai after switching from Toyota, the one-time WRC winner set the fastest time on five of the day’s eight stages to lead six-time Rally Mexico winner Sebastien Ogier by 5.3s.
Lappi found himself locked in an intense fight with Ogier, the driver he alternated Toyota's third GR Yaris with in 2022, as the pair broke away from the chasing pack across Mexico’s challenging gravel stages.
Lappi headed into Friday's midday service with a 1.4s advantage which then grew to 6.9s before Ogier issued a response on the final superspecial stage held at the Leon service park in the evening.
Reflecting on his drive, the Finn admitted he was surprised to be leading the rally.
“I hoped that I could fight at the top but to be leading and fighting all the time against Seb, this was not in my mind to be honest,” said Lappi, who claimed his sole win to date on the 2017 Rally Finland with Toyota.
“On the other hand I didn’t really think about how this day will go, but I was confident our pace would be good.
“I’m a bit surprised but I will take it as it is, and it is probably one of the best days of my career.”
After initially struggling to gel with the i20 N at the season opener in Monte Carlo, Lappi has quickly found the set-up sweet spot, which he puts down to a significant amount of work carried out since the January event.
"It is a good car and we did a a lot of work during the rally in Monte Carlo and we did a lot of work between Monte Carlo and Sweden and during Rally Sweden as well," he added.
"But I think we can stop searching for the good setup as I think we have found what I want and what I like.
"I can now concentrate on driving and that helps."
Lappi’s faultless drive was acknowledged by Ogier, with the Frenchman revealing that there "wasn’t much more" he could do other than take more risks to leapfrog the Hyundai driver.
"It has been a good day and I think we can be happy," said Ogier.
"I don’t think there was much more we could do except taking a lot of risk, but that is not really the approach I want to have on this rally. I’m satisfied.
"So far Esapekka has done a great job, so well done to him. But there are three days on this rally.
"For sure, there is some work we can do to improve. There are some sections where we are definitely not matching the pace, so we have to analyse to see if we can find a bit more."
When asked if he would consider taking more risks, he added: "The fight is on anyway. I think I will keep doing the same and you cannot say we are cruising, we are pushing but not trying to over do it."
Rovanpera happy with damage limitation
Meanwhile world champion Kalle Rovanpera declared himself “pretty happy” to end the day sitting 59.7s adrift in fifth.
Rovanpera started the day second on the road order, although he effectively became the road sweeper for much of morning when overnight leader M-Sport’s Ott Tanak suffered a turbo failure.
"To be honest I’m pretty happy with that," said Rovanpera.
"In previous years there has been even more time lost on the first loop on some stages so we were not so far away today.
"The starting place is better so of course tomorrow we need to try to use it well, and hopefully we can fight with the guys that are close to us."
