Zanz wanted to prove that Antagon wasn't all-powerful because he was incapable of losing.
This implies Zanz wants to prove Antagon can lose.
Antagon smiled and took the bet.
This implies Antagon wanted to win.
Zanz wanted to make sure he would win the bet, so he rigged the Masters tourney duel.
This would imply that he'd give Zeru a huge advantage
Zeru started with 0 HP, 0 cards in hand, and 1 card in deck. Antagon started with 999HP, 7 cards in hand, 999 cards in deck, and a table full of different kinds of Dragons.
This would be a huge advantage in Antagon's favor, which makes no sense.
Zeru beat Antagon 2 - 0.
This implies that despite a huge advantage, Antagon couldn't beat Zeru, and must suck.