Fiala played in the Ivan Hlinka Memorial Tournament in 2012 and 2013 as a member of the Swiss U18 national team. He also skated for his country at the 2014 World Junior Ice Hockey Championships, where he was lauded as one of the key players on the Swiss roster. In five WJC games, he recorded one goal and four assists, as Switzerland was eliminated by Canada in the quarter-finals. In May 2014, Fiala represented Switzerland at the 2014 IIHF World Championship in Belarus, registering two assists in seven games. Fiala was the third player in history to play in the under-18, World Junior, and World Championship in the same year (the first two were Andrei Kostitsyn and Vadim Karaga, both with Belarus in 2003). Fiala also participated at the 2018 IIHF World Championship, where the Swiss won silver. In the final against Sweden Fiala had a chance to clinch the gold in overtime but his shot went too low. After losing the shootout the Swiss had to settle for silver.