With one minute left of the first round against France's Estelle Mossely, the 23-year-old Anastasiia Beliakova threw a punch at an awkward angle with her left hand and then immediately screeched in pain and stopped.
A doctor climbed through the ropes and rushed to her aid, as she stood grimacing in pain and unable to move.
She was coaxed gingerly out of the ring and into a wheelchair, and spirited out of the arena as she cried in distress.
Asked by AFP how she felt about getting into Friday's lightweight final in that way, Mossely said: "It's a contrasting feeling because of course I'm very happy but obviously I would have preferred the win was different, but that's the result.