Joe Prunty's side were trailing by ten points in the second quarter but stormed back, courtesy of Dan Clark's game high 19 points, and started the final quarter deadlocked.
But they were outscored 12-5 in a vital and decisive late spell, the final 76-71 scoreline leaving the team with a 1-2 record ahead of their next match with Germany on Sunday.
"The big thing is understanding that we do have a big game coming up after the rest day," said Prunty.
"We'll get prepared, mentally and physically, we'll rejuvenate.
"We've got a game plan on how to attack. But we'll take advantage of there being a rest day to not only recover from three hard-fought games but also to prepare for two important last games."
Devon van Oostrum, Great Britain's youngest player, put his team 62-60 ahead with six minutes remaining but Belgium's long-time Euroleague star Axel Hervelle and Wen Mukubu, who had four three-pointers, were instrumental in a decisive final push.
"They played a solid game but I'm just disappointed," said Great Britain's Kieron Achara, who netted 12 points.
"I felt like we had the game but we made some mental lapses in the second half and they capitalised on it. You can't make mental lapses like that in a tournament like this."