The Finnish rivals posted identical times through the 5.52km Llano Grande shakedown stage, which marked the first time the FIA World Rally Championship stars have gone against the clock on gravel this season.
Defending champion Rovanperä, driving a Toyota GR Yaris, looked set to take the warm-up test honours after topping the timesheets on his third and final pass. But Lappi, who suffered a fiery exit on his last visit to Mexico in 2020, took his Hyundai i20 N for an additional fourth run and matched his compatriot.
Six-time rally winner Sébastien Ogier was third in another Yaris, just three-tenths of a second adrift of the pacesetters ahead of this evening’s start in León.
“It was what we were expecting,” said Rovanperä, who starts Friday’s opening leg second on the road behind championship leader Ott Tänak. “It was really slippery in the first pass with not much of a clean line.
“For sure, I am going to be suffering a lot [with road position]. It will definitely be hard for us tomorrow. Even though we have Ott in front of us, you still can’t follow the line all the time because it’s just one car.”
Lappi added: "It was a good shakedown. We tried a few things on the second and third runs which really didn't pay off, so on the fourth we went back to the original settings which proved to be good."
Hyundai part-timer Dani Sordo took advantage of the clean line offered by his late starting position to head the leaderboard after two runs. The Spaniard couldn’t keep pace with the frontrunners, however, ending the session a single tenth down on Ogier in fourth.
Just 0.2sec behind was Rovanperä’s colleague Elfyn Evans, who headed Hyundai man Thierry Neuville by 1.3sec. Tänak and his M-Sport Ford Puma team-mate Pierre-Louis Loubet were seventh and eighth respectively, while Takamoto Katsuta and WRC2 driver Gus Greensmith completed the leaderboard.