Jessica von Bredow-Werndl took the Grand Prix Freestyle at the Dressage World Cup Western European League in Salzburg on Sunday (24 January).
For six-time Olympic gold medallist Werth, it was just her second World Cup defeat with Weihegold OLD having won the last three World Cup Finals on the mare.
Bredow-Werndl capitalises on Werth errors
This was the second leg of the Western European League with Werth just beaten by Denmark's Cathrine Dufour in October's opener in Aarhus.
The 51-year-old looked the epitome of calm for the first half of her test with impressive canter and left-hand pirouette sequences.
But the end of her routine did not quite go to plan with the world number one shaking her head afterwards, and her score was her lowest in Grand Prix Freestyle since late 2016.
Von Bredow-Werndl and Dalera defeated Werth and Weihegold in the Grand Prix Special at September's German Championships.
They repeated the trick in Salzburg with the long-striding mare relaxed throughout and handling the transitions into the more intricate movements.
In the end, it was a comfortable victory with von Bredow-Werndl winning by over three points to secure her place in the World Cup Final in Gothenburg, Sweden from 31 March to 5 April.
Werth automatically qualifies for Gothenburg as the defending champion with Langehanenberg in pole position to take the third and final German spot after completing the podium with 81.340 on board Annabelle.
The third and final leg of the Dressage World Cup Western European League is scheduled for 's-Hertogenbosch in the Netherlands in mid-March.