Štefanek represented Serbia and Montenegro at the 2004 Summer Olympics in Athens, as well as Serbia at the 2008 Summer Olympics in Beijing and 2016 Summer Olympics in Rio de Janeiro, where he won the gold medal in the 66kg event. Štefanek was the first Serbian athlete to win a medal in Rio de Janeiro after 11 days of disappointing results and was credited by other Serbian athletes for raising their spirits. The Serbian team then proceeded to win 7 more medals in the following days, making the 2016 Olympic Games edition by far the most successful in Serbia's history as an independent nation until that point.