[2] In 2021, at the delayed 2020 Summer Olympics in Tokyo, Zillekens, under her maiden name Annika Schleu, received widespread attention after her treatment of the horse she was assigned for the riding portion of the event went viral.

[7][8] Her trainer Kim Raisner was sent home from the Olympics after footage showed her seemingly punch Schleu's assigned horse for not performing in a desired manner.

[9] The horse named Saint Boy was randomly assigned from a pool of 18 and had also with Russian Olympic Committee athlete Gulnaz Gubaydullina (32nd overall) refused to jump three barriers in the second half of the equestrian course.

In response to this issue, modern pentathlon's governing body voted to remove horse riding from its programme at the Olympics after the Paris 2024 Games.

[18][19] Modern pentathlon at the 2024 Summer Olympics in Paris will be the last time the sport will include show jumping at an international games.

[25][26] She was also part of the German women's relay team that won the silver medal at the 2014 and 2017 European Championship, alongside Lena Schöneborn.