Irish boxer John Joe Nevin scored a stunning victory over top seed and reigning world champion Estrada Alvarez of Cuba.
Alvarez looked to take advantage of his longer reach in the opening stages but Nevin was up for the challenge and found a way through, notably with a big right hand that rocked his opponent's head back. He took the opener by two points and then edged round two 7-6 despite Alvarez seemingly getting back into the contest with some well-directed shots.
Nevin caused himself some unnecessary anxiety in the final round when he began showboating, shuffling his feet to the delight of the crowd but getting caught with a couple of clean hits for his troubles. He had enough in the bank to render that irrelevant, though, and walked away with a 19-14 success to reach the final. A gold or silver medal now awaits.
He told BBC1: "I am up there with the best in the world and I can be the best in the world if I put my mind to it and get a good performance. My performances are getting better every day and hopefully come the final I'll be ready for the final and get better again.
"It was a good performance. I was up against the world champion so I had to push myself. I'm delighted, I'm over the moon.
"I just want to head in and relax now and think about the next contest. I'll enjoy the night now and there will be another big step but it seems to be I'm getting better as it goes along, so hopefully I can be ready."
Fellow countryman Paddy Barnes has taken Olympic bronze in the light-flyweight division after being beaten at the semi-final stage by China's Shiming Zou for the second Games in a row.
The Belfast fighter was beaten 15-0 by Zou in Beijing four years ago and despite a vastly-improved showing in the ExCeL Arena, he could not exact revenge, defeated on countback after the judges scored the bout level at 15-15.
Barnes slipped 8-5 behind in the first bout but Zou backed off thereafter, tieing the next round 3-3 and then being soundly beaten in the third as Barnes connected with a series of rights and lefts as he went all out to cut the deficit. He successfully did that, but not enough to progress to the gold medal match.