Amnat Ruenroeng was halted by 21 year-old amateur Lindolfo Delgado in Vargas, Venezuela as the Thai fighter lost out on the gold medal.
Ruenroeng, 36, had already qualified for the Rio 2016 Olympics when Delgado took him out to take the 60kg title.
A world title holder at 112 pounds, Ruenroeng is campaigning 20 pounds higher than usual in his quest for glory at the summer games in Brazil but will be a lot more cautious in the next phase after being humbled by Delgado.
In another shock, former middleweight world champion Hassan N’Dam lost on points to Juan Carillo of Colombia in what was essentially a light-heavyweight bout.
N’Dam booked his spot on the plane to South America with a previous triumph in the semi-final, although will need to up his game to have a chance of a medal in the coming weeks.
Other winners at the event, included Ceiber Avila of Colombia, Carmine Tommasone of Italy and Edgar Munoz of Venezuela.
In total, 26 boxers made it through to the Olympic event which begins on August 6 and ends August 2 in the following categories:
Light flyweight (49kg)
Light welterweight (64kg)
Light heavyweight (81kg)
Super heavyweight (+91kg)