Mašek won ten medals at the ICF Canoe Slalom World Championships with a gold (C1 team: 2002), two silvers (C1 team: 2006, 2014) and seven bronzes (Mixed C2: 2017, 2018, 2019, C1 team: 2003, 2005, 2007, 2010). He also won 6 medals in the C1 team event at the European Championships (2 golds, 2 silvers and 2 bronzes).