Competing in the −94 kg category, Zieliński won silver at the 2014 European Championships in Tel Aviv and gold at the 2016 European Championships in Førde. He was included in Poland's squad to the 2016 Summer Olympics after the national weightlifting federation's board over-ruled its own president, Szymon Kołecki, who had wanted him off the team after Zieliński decided to train outside the federation's framework. At the Olympics in Rio, Zieliński tested positive for spironolactone and was sent home.