Deas was second and Yarnold, who won gold at the Sochi 2014 Games, third in the first official training run on day three of the 2018 Winter Olympics.
And in the second run Deas was fastest, with Yarnold fourth quickest, results which bode well for the four-run competition that begins on Friday and concludes on Saturday.
Austria’s Janine Flock finished her first run in 52.35 seconds, 0.01secs ahead of Deas and 0.13 ahead of Yarnold.
Deas clocked 52.45 for the second run, with Flock second, 0.02 behind, and Yarnold 0.27 adrift in fourth.
Deas, a 29-year-old from Wrexham, claimed her first World Cup win in November 2015 and was fourth at the 2017 World Championships.
The last two women’s skeleton titles have gone to Britain with Yarnold succeedinig Vancouver 2010 winner Amy Williams as champion.
Shelley Rudman won Olympic silver in Turin in 2006, while Alex Coomber took bronze in Salt Lake City four years earlier.
In men’s training, Britain’s Dom Parsons was sixth on the first run and first on the second, clocking 50.74, 0.32 clear of his nearest rival.
Fellow Briton Jerry Rice was 14th and eighth in his two runs.
South Korea’s Yun Sung-bin, the World Cup champion after dominating the circuit, did not take part on Monday.
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