Cuban boxer Odlanier Solis will face Viktar Zuyev of Belarus in the Olympic
heavyweight final on Saturday.
Solis stopped Syria's Naser Al Shami with 31 seconds remaining in the third round, after Al Shami had received a standing count of eight in the first.
Zuyev progressed without fighting after his opponent, Egypt's Mohamed Elsayed, failed the pre-fight medical.
Meanwhile, Thai boxer Manus Boonjumnong made it to the light welterweight final after beating Romania's Ionut Gheorghe.
Manus was in great form, using his left jab to help him to a 30-9 victory.
The Thai will fight Cuban Yudel Johnson for gold after he defeated Boris Georgiev of Bulgaria 13-9.
Johnson put Georgiev on the canvas 20 seconds into the fight and grabbed his win when the Bulgarian received another standing count in the third.
Elsewhere, Turkey's Atagun Yalcinkaya upset Russia's world champion Sergey Kazakov in the light flyweight category.
Yalcinkaya, who is six days younger than lightweight finalist Amir Khan of Great Britain, is hoping to become the youngest Olympic boxing champion since American Floyd Patterson in 1952.
The Turk faces Cuban Yan Bhartelemy for the gold medal.
The other big upset of the day was staged by Kazakhstan's Bakhtiyar Artayev, who ended Russian welterweight Oleg Saitov's quest for a rare Olympic treble.
Saitov was trying to become the fourth boxer to win three Olympic gold medals and the third to do it in the same division after Cuban heavyweights Teofilo Stevenson and Felix Savon.
The United States boxing team only have one chance to win a gold medal, after middleweight Andre Dirrell lost 23-18 to world champion Gennadiy Golovkin of Kazakhstan.
The USA will be hoping light-heavyweight Andre Ward can win his semi-final against Utkirbek Haydarov of Uzbekistan, later on Friday.
Cuba's Yuriorkis Gamboa will face France's Jerome Thomas in the flyweight final with a 20-11 win over Rustamhodza Rahimov of Germany.
Thomas, who beat Azerbaijan's Fuad Aslanov for a 23-18, is confident of beating Gamboa.
"I've never faced a Cuban in a senior event but they have two arms and two legs like everybody else," he said.