Despite having limited testing time prior to the event, Greensmith appeared well-acquainted with his Toksport-prepared machine. He consistently posted top-three stage times throughout the morning and reached the mid-leg service just 3.0sec down on overnight leader Emil Lindholm, who drives a Fabia Rally2 Evo.
It put the Brit in prime position to pounce in the afternoon when Lindholm dropped 14 seconds on the second passage of El Chocolate, demoting the Finn four positions and leaving Greensmith on top, where he remained for the rest of the day.
"The morning was a little bit tricky because the tyres were overheating, but I knew that once we put the hard tyres on I could go much quicker," Greensmith said. "It's a nice lead, but not a massive one. It's all about managing risks and the pace here, and trying to be tidy."
Lindholm's demotion lifted Adrien Fourmaux to second in his Ford Fiesta Rally2. The Frenchman - a former team-mate to Greensmith at M-Sport - spent the majority of Friday locked in a tight battle with Oliver Solberg, but it was the Swede who flinched first.
Just 0.7sec separated the pair before the second running of Ortega, where Solberg dropped more than 1min 30sec with a front-right puncture. He slipped to fifth, giving Fourmaux a more manageable 12.2sec advantage over Lindholm, who subsequently moved back into podium position.
Splitting Lindholm and Solberg in fourth overall at close of play was Kajetan Kajetanowicz. The Pole, who is contesting his first rally of the season, trails Lindholm by 36.6sec overnight. Paraguay's Diego Dominguez leads the WRC3 category by over four minutes from Ford Fiesta Rally3 rival Jason Bailey.