Fellner, who starts for the Fenix Ski Team Jesenik, took part in the Slavic Cup for the first time in Horní Mísečky in December 2010 and finished 52nd over 10 km classic. He made his debut in the World Cup in February 2012 in Nové Město , which he finished in 62nd place in the 30 km mass start race. At the Nordic Junior World Ski Championships in 2013 in Liberec , he came in 60th place in the skiathlon , on 54th place over 10 km freestyle and on 13th place with the relay. His best results at the Winter Universiade 2013 in Lago di Tesero were 40th place over 10 km freestyle and fifth place with the relay. In the 2013/14 season he achieved two top ten finishes, including second place over 10 km freestyle in Štrbské Pleso , eighth place and in the 2014/15 season he achieved three top ten finishes, including third place over 15 km classic in Nové Město, ninth place in the overall ranking of the Slavic Cup. At the end of January 2015, he won the bronze medal with the relay at the Winter Universiade in Štrbské Pleso. He also achieved 31st place over 10 km classic and 14th place in the 30 km mass start race. The following year he ran at the U23 World Championships in 2016 in Râșnov on 45th place in the sprint, 27th place over 15 km freestyle and 26th place over 15 km classic. In the 2016/17 season he finished in the top ten in all six participations in the Slavic Cup and thus won the overall ranking. He achieved third place over 10 km classic in Jablonec nad Nisou and took his first victory in the Slavic Cup in the pursuit race the next day. At the 2017 Winter Universiade in Almaty , he finished 20th in the 10 km classic and 12th in the 30 km mass start race. At the Nordic World Ski Championships 2019 in Seefeld in Tirol , he ran to 51st place in the skiathlon, 49th place over 15 km classic and 11th place with the relay. In the 2019/20 season, he scored his first World Cup points with 47th place in the 2019/20 Tour de Ski .
Czech Republic