Antonopoulos qualified for the men's 4 × 200 m freestyle relay, as a member of the host nation team, at the 2004 Summer Olympics in Athens. Teaming with Nikolaos Xylouris, Andreas Zisimos, and Dimitrios Manganas, Antonopoulos swam a lead-off leg in a split of 1:50.34, but the Greeks rounded out the finale to last place with a final time of 7:23.02, more than 15 seconds off the winning mark set by Team USA.