Everton dos Santos Lopes was eliminated in his opening bout by Beijing bronze medallist Roniel Iglesias Sotolongo of Cuba.
Lopes became the first Brazilian to win a world championship title last year but he was comprehensively out-boxed and outsmarted by the taller, quicker Cuban, who picked him off with sharp one-two combinations to take the first round by a point and the second by three.
Unable to get near Sotolongo, the 23-year-old Brazilian admitted it was too late by the time he changed his approach in the final round, winning it by one point after trying to land more right than left hand shots.
"I knew he was a very good opponent. When I saw I was losing I tried to change my technique to win but it wasn't enough at the time. I should have done it before," Lopes told reporters.
"I wasn't expecting to fight him (so early in the competition) because I know he's so strong but I did my best to try. I know what we (Brazil) have to do is to try because we are good and we know we are good."
With the 2016 Rio Games on the horizon, Brazil had high hopes for Lopes in London, and will now hope the three men and three women left in the competition will add to the country's one and only Olympic boxing medal.
Sotolongo, who won Olympic bronze at the age of 19 in a disappointing Games for Cuba four years ago, next faces Uktamjon Rahmonov of Uzbekistan in Wednesday's quarter-final.
As both losing semi-finalists get a bronze in Olympic boxing, Sotolongo would at least guarantee matching his Beijing result with a win.
Defending Olympic champion Zou Shiming of China survived a tricky opening bout, meanwhile, and had to call on all the know-how that delivered three world amateur titles in seven years to edge another Cuban, Yosbany Veitia Soto.
Fleet-footed Soto, who impressed on his way to the quarter-finals of the world amateur championships last year, was the busier fighter throughout and the 20-year-old delivered a serious challenge to Zou's light-flyweight dominance.
But the seasoned Zou, almost 11 years Soto's senior, stayed calm and landed one opportunistic punch after another, winning each round by a single point for a 14-11 victory.
"As number one, there is always pressure but pressure brings out the fighting spirit," Zou, who fights Birzhan Zhakypov of Kazakhstan next, told reporters through a translator.
Zhakypov, a bronze medallist at the world championships in 2005, came from two points down going into the final round to edge out Mark Barriga, the Filipino fighter nicknamed 'Little Pacquiao' after his country's boxing great Manny Pacquiao.
If Zou wins that fight, he could set up a rematch of his 2008 Beijing semi-final against Ireland's Paddy Barnes, who four years ago left the Olympics stunned that he did not score a single point in a 15-0 loss to home favourite Zou.
Barnes, who eased to a 15-10 win over Cameroon's Thomas Essomba and has India's Devendro Singh Laishram next, has had a long wait to make his London Games bow.
And while his team mates have been chasing famous Olympians around the athletes village for their autographs to see who can collect the most the Belfast man said he had been harassed for his signature by some big-name sprinters.
"I've been walking around the village getting chased by all these athletes," he told reporters, tongue firmly in cheek. "(Usain) Bolt, (Asafa) Powell and Tyson Gay chasing me for autographs and pictures. I had to stop giving them.