Gabe Olaseni led the scoring for GB with 13 points, Dan Clark added 12 and Kavell Bigby-Williams 11 as they showed tenacity to stay in the game.
Greece established a double-digit lead at the end of the first quarter and then held a spirited GB team at bay.
After trailing 66-51 at the end of three quarters, GB scored 13 points in a messy final quarter in Patras.
Greece are one of the strongest teams in Europe, even without their NBA star Giannis Antetokounmpo, whose knee injury kept him out of the tournament.
The hosts scored 15 of the first 20 points in the game, but GB trailed 17-21 at the end of the first quarter as newcomers Bigby-Williams and Jules Dang Akodo combined for six consecutive points.
Greece took control of the game in the second quarter with a 16-5 run before Clark's shooting brought GB back from 26 points adrift to 51-34 down at the interval.
The deficit at the end of the third quarter was similar (66-51) but any thought of a comeback ended when Greece scored the first nine points of the fourth period and cruised to a slightly flattering victory as GB ran out of steam.
GB's next game comes on Sunday, 13 August when they play Israel in Tel-Aviv, before facing Greece again in London in a friendly at the Copper Box in London on Saturday, 19 August.
GB head coach Joe Prunty said: "I think we had our opportunities where we got the lead down to maybe 14. I thought we competed. [The young players] are coming in and proving they want to be part of it, and there's a lot of people who do, so you have to give the people who are getting their first caps a lot of credit."
GB top scorer Olaseni added: "It's just development and progression -the effort was there, we just need to execute a little better. My team-mates do a great job of getting me open, I just have to be able to finish."
Hosts Greece comfortably beat Great Britain 92-64 in the final of a warm-up tournament for EuroBasket 2017.