Deicmans qualified for two swimming events at the 2004 Summer Olympics in Athens, by eclipsing FINA B-standard entry times of 2:03.77 (200 m individual medley) from the FINA World Championships in Barcelona and 4:28.44 (400 m individual medley) from the European Championships in Madrid. In the 400 m individual medley, Deicmans touched out Philippines' Miguel Molina to break a 4:30 barrier and capture a seventh spot on the second heat by four seconds, in a time of 4:29.17. In the 200 m individual medley, Deicmans challenged seven other swimmers on the fourth heat, including two-time Olympians Jeremy Knowles of the Bahamas and Ioannis Kokkodis of Greece. He cruised to fifth place by 0.39 of a second behind Barbados' Bradley Ally, breaking a Latvian record of 2:03.68. Deicmans failed to advance into the semifinals, as he placed twenty-fifth overall in the preliminaries.