Antonius began making a name for himself on the poker tournament circuit with two finishes near the final table of a European Poker Tour (EPT) event and a World Poker Tour (WPT) event, 12th at the EPT PokerStars Caribbean Adventure, then 15th at the WPT Bay 101 Shooting Stars event two months later, in early 2005. He went on to finish in the money in three events of the 2005 World Series of Poker (WSOP). In September 2005 he made the European Poker Tour (EPT) Main Event final table, finishing 3rd in Barcelona. The next month, Antonius won the EPT event in Baden bei Wien, taking home the €288,180 first prize when in the final hand his beat Gunnar Østebrød's on a board of . In December 2005 he finished the year 2nd in the WPT Five Diamond World Poker Classic in Las Vegas, Nevada, winning $1,046,470.