Ford's Jari-Matti Latvala has slipped back in Mexico after suffering a front suspension problem in SS5.
Although details are currently sketchy, it is known the Finn conceded over 90 seconds in Ortega 1 and he now drops out of contention.
Latvala, who had taken the lead in SS3, when he set the quickest time, had been 2.2 seconds up on Mikko Hirvonen after Los Mexicanos 1. However his misfortune, hands P1 to his former team-mate.
Hirvonen will now go into the mid-day service 0.6 seconds up on Sebastien Loeb, with Citroen well placed at this early stage for a 1-2.
Mads Ostberg is third, albeit 39.3 seconds adrift, while Thierry Neuville, Ott Tanak and Evgeny Novikov round out the top six.
Latvala's Ford team-mate Petter Solberg is seventh, after he lost his overnight lead with a puncture in SS2, the first 'proper' test of the day. Not a great start to the weekend for Ford...