Sordo, at the wheel of a Hyundai i20, came up from 10th position with Loeb just a whisker behind on the dusty trails at altitude in the central city of Leon.
“After six years away, it’s good to be in this position against such a strong opposition,” Loeb said after stage two.
The Frenchman will take part in just three races this season.
“I’m in the lead but it’s not comfortable,” Sordo said after the second stage. “Sébastian and Ott [Tanak of Estonia] are pretty strong behind. I’m impressed by Sébastien but he’s not nine-time world champion for nothing.”
Estonia's Ott Tanak was third in his Toyota, 11 seconds off the leaders.
Thierry Neuville, from Belgian who heads the world championship standings after the first two races, is back in seventh spot, two minutes 1.9 seconds behind the leader, after the stage.
They have however avoided the fate of Britain's Elfyn Evans and the Finns Esapekka Lappi, Teemu Suninen-Mikko Markkula and Jari-Matti Latvala (Toyota) who were all forced out of the race.
Evans's co-driver Daniel Barritt was hospitalised with from concussion after their Ford Fiesta WRC rolled at high-speed.