No matter how distracted Dill is, there is no god damn way that he would struggle that much against an opponent that he was shown to be able to dispatch that easily. This would be a little more believable if Dill's strength wasn't too far apart (just surpassing by a small margin) from his opponent's. (but only a little)
The only way such a terrible performance would be viable is if the opponent directly threatened the outcome of the match with Ain's safety. Like "if you win this match, Ain will be killed. I'll give you the duration of the match to think about your decision" and Dill fumbles until Ain rushes into the colosseum, confirming that he isn't being held hostage.
As it is within the story right now, that was fucking horrible.