Koristek started in Kremnica for the first time in the Slavic Cup in March 2012 and finished 42nd place over 15 km freestyle and 39th place over 10 km freestyle. At the European Youth Olympic Winter Festival 2013 in Predeal he came in 74th place in the sprint and 30th place over 10 km freestyle. In the following year, at the Junior World Championships in Fiemme Valley, he achieved 79th place in sprint, 61st place in skiathlon and 38th place over 10 km classic. At the Junior World Championships 2015 in Almaty he ran to 71st place in sprint, 55th place over 10 km freestyle and 49th place in skiathlon. In the 2015/16 season he reached eighth place in the overall ranking of the Slavic Cup with four top ten placements. At the Junior World Championships in 2016 in Râşnov , he ran to 72nd place in the sprint, 45th place over 10 km classic and 29th place over 15 km freestyle. In the 2017/18 season he finished in the top ten three times in the Slavic Cup. He achieved his first podium in the Slavic Cup with second place over 15 km freestyle in Wisła and ninth place in the overall standings at the end of the season. At the U23 World Championships in Goms in 2018 , he achieved 69th place in the sprint and 54th place over 15 km classic.
At the beginning of the 2018/19 season, Koristek completed his first World Cup race in Ruka , which he finished classically in 79th place over 15 km. In the following Lillehammer Triple, he ran to 76th place. In the further course of the season he won four Slavic Cups. He also came third twice and thus won the overall standings. In January 2019, he finished 23rd in the 30 km mass start race and 17th place in the 15 km freestyle at the U23 World Championships in Lahti . At the season highlight, the Nordic World Ski Championships 2019 in Seefeld in Tirol , he achieved 61st place over 15 km classic and 22nd place together with Andrej Segeč in the team sprint. In March 2019, he became Slovak champion in the 30 km mass start race in Štrbské Pleso . His best placement at the Winter Universiade 2019 in Krasnoyarsk was eighth in the 30 km mass start race. After finishing 70th in the Ruka Triple at the beginning of the 2019/20 season, he picked up his first World Cup points with 53rd place in the 2019/20 Tour de Ski . This was followed by third place in the sprint at the Slavic Cup in Strbske Pleso and second place over 15 km classic and at the end of the season fifth place in the overall ranking of the Slavic Cup. In March 2020 he won the Biela Stopa over 37 km freestyle.
Slovakia
Zakopane