Liess qualified for the men's 200 m butterfly at the 2012 Summer Olympics in London, by breaking a Swiss record and eclipsing the FINA B-cut off time of 1:58.13 at the European Championships in Debrecen, Hungary. He challenged seven other swimmers in the second heat, including former semifinalist Hsu Chi-chieh of the Chinese Taipei. Liess was in last place by 0.16 of a second behind Canada's David Sharpe. Liess failed to advance into the semifinals, as he placed thirty-third overall in the preliminaries.