Swiss Mueller, who took her first career win at Bischofswiesen last weekend, paced the session, 0.02 seconds ahead of Canada's Marielle Thompson and Norwegian Hedda Berntsen.
Ophelie David of France completed the top four for the women.
In the men's competition, Schmid, also of Switzerland, tied Canadian Del Bosco for top spot, despite falling and sliding across the line in a prone position.
Jouni Pellinen of Finland and Tomas Kraus of the Czech Republic rounded off the top four.
Switzerland's Alex Fiva leads the race for the men's Crystal Globe while David holds a 30-point advantage over Thompson on the women's side.
The finals are scheduled for Saturday afternoon in Branas.