Greece led 75-60 midway through the fourth quarter, but Italy (1-1) got as close as 84-81 on a layup by Simone Fontecchio with 35 seconds left.
But Italy's comeback stalled there. The Italians missed two potentially game-tying 3-pointers in the final 7 seconds and Dorsey made a free throw with two-tenths of a second remaining to seal the win.
Fontecchio led Italy with 26 points. Achille Polonara scored 14 points and grabbed 10 rebounds, and Stefano Tonut scored 13.
Bojan Bogdanovic shoots 6 for 11 in 18 minutes of action.
CROATIA 86, GREAT BRITAIN 65
Croatia outscored Great Britain 35-9 in the third quarter, building a 35-point lead, and cruised to a win.
Bojan Bogdanovic, Dario Saric and Ivica Zubac each scored 15 for Croatia (1-1), while Mario Hezonja finished with 13 points and 10 rebounds.
Myles Hesson scored 18 and Patrick Whelan finished with 14 for Great Britain (0-2).