The Frenchman, who will retain his crown today if he finishes in the points and ahead of Ford's Mikko Hirvonen, ended the penultimate day with a 30.2 second advantage over the second-placed Finn.
Hirvonen had cut Loeb's lead to just 2.9 seconds after a wet and muddy opening stage of the day in South Wales but was powerless to prevent his rival roaring ahead after that.
With just four stages remaining, Loeb looked in complete control while Hirvonen will have to go flat out and take plenty of risks.
The Finn had a one-point lead in the championship ahead of the season-ending race but Loeb has more victories to his credit.
"The rain this morning made the roads good for me. I had plenty of grip," Loeb told the official wrc.com website at the midday interval.
Hirvonen, chasing his first title, was bewildered by his car's lack of pace and worried that he could be developing similar driveshaft problems to the ones that sidelined his team mates on Friday.
"For some reason I just can't do the times," he said. "I'm trying but it's just not going anywhere.
"In some places the car feels like it's not turning in and it's oversteering. It could be the driveshaft. I don't know.
"I have a lot to do now. I have to keep pushing, that's for sure."
The rally ends in Cardiff today.