Fernstädtová's first season on the World Cup was 2016–17, when her best finish was a bronze at Königssee; that season, the world championships were held on the same Königssee track and Fernstädtová finished fourth, but earned a bronze as part of an international squad in the mixed team competition. She started the 2017–18 season on the ICC, but was promoted to the World Cup squad in the place of Sophia Griebel after winning three ICC races. She took bronze in the 2018 race at Altenberg, and won the 2018 Junior World Championships by 0.81 seconds, ahead of 2017 champion Yulia Kanakina.