Yarnold, consistently the fastest in training, was quickest in both the opening runs, meaning she will take a 0.44 second advantage into the third and fourth run over American Noelle Pikus-Pace, her closest rival all season.
Four years ago Amy Williams took a 0.30 second lead into the concluding runs and Yarnold will now look to follow her landlady's lead - and secure Team GB's fourth consecutive Olympic skeleton medal on Friday.
"I don't feel the pressure, I've not been thinking about other peoples expectations because I've got such high expectations of myself anyway," she said.
"I was relaxed as soon as I got to the track. As soon as I stepped on to the start line my coach really settled me and once I started everything felt so natural and I loved it.
"I've been trying lots of different things in training and I've brought that through into competition.
"The first and second run are the fastest I've ever gone. It's as good as it gets really."
Yarnold has been totally dominant this season, claiming seven World Cup podiums in eight races, including four wins, to arrive in Sochi ranked number one and the slider to beat.
But British Skeleton's performance director Nigel Laughton was insisting nothing is won yet - while admitting his admiration for Yarnold's attitude under the most intense pressure of her career.
"Lizzy has done brilliantly well, that's two very solid runs and it's a nice place to be," he said.
"In this sport it's a good lead but it's not insurmountable and she needs to be on the mark tomorrow. She's an ultimate professional and she will come out with everything to prove."
Team-mate Shelley Rudman, the reigning world champion, has been struggling with the track at the Sanki Sliding Centre and corrected an untidy first run with an improved second attempt, leaving her ranked 11th overall.
"I didn't get curve 14 on the first run and it's an uphill section so that bled my time," said Rudman, an Olympic silver medallist eight years ago in Turin.
"I pushed a lot faster for the second run and made up a bit of time, so I can only be pleased with that. It's not a bad day."