Ahlmann, 2011 World Cup winner, led the way as nine other riders cleared the first round at the course designed by another German Franck Rothenberger.
Among those to clear were Olympic champions Marc Houtzager, Gerco Schroeder and Maikel van der Vleuten, twice Global Champions Tour winner Edwina Tops-Alexander and world number five Kevin Staut.
But Ahlmann, sixth to start, was in a different class with a near-perfect round that showcased a beautiful harmony between rider and horse.
He finished with a time of 36.45 seconds, while Denis Lynch atop Abbervail van het Dingeshof came in second with 38.65.
Schroeder, riding London, took third with a time of 39.53 and, like Lynch, was able to move a step closer to the finals in Gothenburg.
- Sports & Recreation
- Marc Houtzager