I'm confident more will be available in the 24 hours leading up to the game. 90% of people haven't arrived yet so that's thousands of tickets potentially available. Also any people with them know most people aren't there yet and will be keeping hold until there's more customers.
I get there on Tuesday early hours and I'm going to be in athens from 6am onwards on the streets looking to get sorted before Dinner and before I even check into my hotel if I can. Not arsed. Just want a ticket to see this football match full stop.