Sedlak's major achievements and milestones include his victory at the Serbian Chess Championship in 2010, the victory at the 12th Dubai Open Chess Championship in 2010, and winning the Serbian Championship in 2013. Additionally, he played in the Chess World Cup in 2011 but was eliminated in the first round.
Nikola was part of the national team representing Serbia in a number of prestigious team competitions, including the European Team Chess Championship and the Chess Olympiad.
In addition to playing, Sedlak is also known for his work as a chess coach. He trained the national women's team of Iran from 2014-2016 and the women's team of India from 2017 to 2019 where they achieved considerable successes.