Welterweight Fred Evans scraped through his quarter-final bout to make it five guaranteed medals for Great Britain in the boxing ring.
However, flyweight and fellow Welshman
was outclassed by Cuba's Robeisy Ramirez.
Great Britain have now equalled their best-ever post-war medal haul in boxing, set in Melbourne in 1956.
could make it six medals when he boxes on Wednesday.
made it three guaranteed medals for Ireland by winning a hard-fought contest against France's Nordine Oubaali 22-18.
Amir Khan's analysis
"Winning by countback is always close. But I think he won because he was the braver fighter and the most pressurising fighter in the first two rounds. He slowed down in the third but he has to work on that. The first two rounds he was coming forward, using his jab and hitting him with some powerful shots. It was a brilliant fight."
Evans, 21, looked very assured against Canada's Custio Clayton, controlling the fight behind his punishing lead right and landing with some snappy right hooks to lead 7-2.
But Custio, looking to win his country's first boxing medal since 1996, narrowed the deficit to two points after the second as Evans, who has a reputation for losing concentration, lowered the work-rate.
And the Cardiff southpaw, presumably thinking he already had the fight sewn up, stopped boxing completely in the third, meaning the judges scored it 14-14.
However, Evans was awarded the fight on countback, and will become the first Welshman to win an Olympic boxing medal since Ralph Evans in 1972.
"I was really comfortable after the first but perhaps I stepped off the gas a little," said Evans, the reigning European champion.
GB's Selby outclassed by Cuban teenager
"I stepped off a bit too much in the second and, when I went back to my corner, they said 'what are you doing? You're stepping off completely'.
"But I always felt comfortable and didn't realise how much he'd pulled it back until I looked at the scorecards, I thought I was three or four up."
The Canadians lodged an unsuccessful appeal against the result on the grounds that Evans should have been deducted points for persistent warnings.
Evans fights world champion Taras Shelestyuk in the last four after the Ukrainian beat Frenchman Alexis Vastine, also on countback.
Shelestyuk looked slightly embarrassed when his arm was raised and Vastine sunk to the canvas - the French team immediately lodging an appeal.
"He's the world champion and I'll have to up my game again to change the colour of my medal," said Evans of Shelestyuk.
What is 'countback'?
Countback is a form of scoring used to decide a fight when the scores are tied at the end of an amateur boxing bout. At London 2012 there are five ringside judges - the highest and lowest scores for each boxer are ruled out and the rest of the scores are added up. The highest total wins.
"That was a little bit of a lesson for me this evening, I'll have to fight like I did in the first round all the way through."
The first round between Selby and Ramirez was fast and furious with the 19-year-old Cuban just getting the better of some scorching exchanges to lead 5-4.
But Ramirez stretched his lead to five points after the second, with the Barry switch-hitter unable to pierce his rival's tight guard and repeatedly being caught on the counter.
That left Selby, 23, with an almost impossible task in the final round - and so it proved, the Pan American Games gold medallist, who won his first two fights in London 19-7 and 22-10, again running out a deserved 16-11 winner.
"He was the better man on the day, a very clever boxer," said Selby, the European champion and World Championship silver medallist.
"I was throwing combinations and my trainer was telling me to put my hands up after I finished and he would just get me before I could get them back up. I could have boxed more and not got involved, because he was very strong.
GB's boxing progress
Andrew Selby - knocked out in quarter-final
Luke Campbell - into semi-finals
Joshua Taylor - lost in third round
Tom Stalker - into quarter-finals
Fred Evans - into semi-finals
Anthony Ogogo - into semi-finals
Anthony Joshua - into semi-finals
"I would have liked to have come away with the win but it didn't come off and my mum said, win or lose, she's the proudest mother there is."
Selby said he would now go on holiday before returning to the English Institute of Sport in Sheffield and discussing his future with performance director Rob McCracken.
Conlan seemed fortunate to be ahead after the first round, during which his all-action French opponent caught him with some solid left hands.
It was more of the same in the second, with the upright Conlan struggling to get his shots off as Oubaali kept coming forward, but again the judges saw it level.
Conlan started to find his mark in the third as the Frenchman began to tire and finally ran out a 22-18 winner.
World flyweight champion Misha Aloyan of Russia used his left cross to great effect in beating Puerto Rico's Jeyvier Cintron 23-13.
Aloyan, the top seed, meets Tugstsogt Nyambayar in his semi-final after the Mongolian upset fifth seed Jasurbek Latipov 15-10.
No United States male boxer will win a medal for the first time ever after welterweight Errol Spence was beaten by Russia's Andrey Zamkovoy 16-11.