Making a 55 minute I/D connection in ATH certainly is possible. I did it a few years ago flying ATL-ATH and then on to Santorini.

To improve my chances, I jammed everything into a carry-on and had no checked luggage. I got a seat near the exit, allowing me to beat most of the people on my flight to passport control. Once I cleared immigration, I knew exactly where to go to check-in for my connection.

If you must check/retrieve luggage, if your inbound is late, if you're among the last people off the inbound flight or if you face long customs lines, your chances would be diminished.

A safer strategy might be to book the later flight and try for the 10 am if all factors cut your way.

Assuming an hour for arrival, and check-in an hour in advance of the later flight, plus travel time to/from the city, you would have very little opportunity to do much in Athens.