I have been trying to find information regarding serious emergencies, that may require more than Hydra services are capable of. The US embassy has a good list of Doctorsand hospitals in Athens @  http://athens.usembassy.gov/us_citizen/medical.htm

They also have a list of Doctors in rural areas, but I don't think any of them are closer than Athens http://athens.usembassy.gov/us_citizen/medical2.htm

I also found Air Ambulances for US citizens traveling abroad: www.americanairambulance.com