Loeb had appeared content to tread carefully through the extremely wet and muddy opening stages of the event.
But when conditions became a little more palatable on the second run through the 44 kilometres of Terra Alta, Loeb dug deeper and produced a stage win that thrust him from fourth to second, 25s down on Ostberg.
Before that, the Adapta Ford driver had been extending his lead, storming through the afternoon's first stage as his morning combatant Ott Tanak began to drop back.
Loeb might have expected to trim a little more from Ostberg's cushion on the mostly asphalt Salou superspecial that brought leg one to a close, but instead he lost two seconds as rain began to fall before he took the start.
Given Loeb's illustrious asphalt record, Ostberg's chances of hanging on to the lead for the rest of the weekend appear remote.
Ostberg conceded that the 49s margin he holds over third-placed Jari-Matti Latvala was his main focus, yet wouldn't completely dismiss his chances of defeating Loeb.
"It's been extremely difficult today. But we seemed to cope with extremely difficult conditions very well," said Ostberg.
"I'm very happy with the day and I'm looking forward to doing some racing tomorrow.
"There are two more long days and if everything is perfect it is even possible to keep Loeb behind. But I reckon it will be hard to do two perfect days on tarmac."
Latvala moved into third as Mikko Hirvonen and Tanak fell back. A wild moment on Terra Alta cost Hirvonen, who rebounded by winning Salou, while Tanak was partly cautious and partly plagued by brake problems following a huge spin.
Evgeny Novikov gave DMACK a superb stage win on Pesells, only to then put his Autotek-run Ford into a ditch on the following Terra Alta stage.
Jarkko Nikara and Sebastien Ogier also hit trouble while in line for top-six slots. Nikara's Prodrive MINI sustained a puncture and had a troubled tyre change, leaving the Finnish World Rally Car debutant eighth. Ogier's Volkswagen-run Skoda Fabia S2000 stopped on the road section before Salou.
SWRC leader Craig Breen therefore holds an astonishing sixth overall, just ahead of Hans Weijs in the Qatar Citroen. Breen's class rival PG Andersson is several minutes behind, but still running in ninth, after a driveshaft breakage in the morning and then an error in the afternoon.