Williams, 28, took a break from competition after winning gold at the Vancouver 2010 Winter Olympics almost a year ago.
With a poor start position based on her lack of World Cup points to date this season, Williams was 17th after the first run but produced the third fastest descent of the day in her second run to climb into 12th overall.
She said: "I'm a bit disappointed, but it's what I expected and predicted.
"I can't remember the last time I was that low in a race. I was last off in the first run, and the ice had really slowed down in the 45 minutes since the first person went.
"It was really sticky.
"It takes a while to get fully back into the race routine.
"I felt much more like myself on the second run. I was really pleased with the way I drove it. I could feel the difference and it was good to move up so many places.
"I've got to think of the second run and be positive.
"I've still got that drive inside me.
"I want to be better and I know I can be."
Her University of Bath-based British Skeleton team-mate Rudman continued her fine season by taking silver behind German's Anja Huber, while Donna Creighton came sixth.
January 2011: Williams back on the ice as she prepares for first event since Vancouver
December 2010: Exclusive - Williams vows "I'm not done yet" as she pledges return in January
November 2010: "Shattered" Olympic champion Williams not ready to return
September 2010: Amy Williams - Letting my hair down at the British Olympic Ball
August 2010: Exclusive - Williams refuses to commit to defending her title at Sochi 2014 Winter Olympics