Frazer Clarke was cut by head clashes which resulted in the disqualification of his rival Mourad Aliev – meaning Team GB’s super-heavyweight is into the Olympic semi-finals.
Clarke is guaranteed at least a bronze medal after a bizarre conclusion to his battle with France's Aliev.
The fight was stopped in the second round with Clarke cut above the eye.
Aliev, who had previously been warned once for excessive use of his head, was disqualified by the referee for a further headbutt.
The fuming Aliev sat angrily on the ring apron for 30 minutes after the unusual ending to his Olympic dream but Clarke marches into the last four.
Clarke will next take on Uzbekistan's Bakhodir Jalolov, the No 1 seed. Jalolov has already won eight professional fights by knockout.
"It doesn't sit well with me, personally," Clarke previously told Sky Sports of professionals being allowed into the Olympics.
"I am old school - you are an Olympian, an amateur boxer. But the world is changing. I'm not saying it's right or wrong. But for me? You make the sacrifice and the stepping stones. This is the apprenticeship then you go into professional boxing. That's how it's always been."
Team GB's Ben Whittaker and Pat McCormack will fight for gold medals after winning their semi-finals.
Light-heavyweight Whittaker outclassed Russia's Imam Khataev in a hugely impressive showing.
Welterweight McCormack was given a walkover after semi-final opponent Aidan Walsh of Ireland withdrew with an injury.
Team GB equalled its boxing medal haul from London 2012, which included Anthony Joshua's gold, with the three results on Sunday morning.