Zou Shiming claimed China's first Olympic boxing medal by beating Armenia's Aleksan Nalbandyan 20-12 in the light flyweight quarter-finals.
Zou now faces Yan Bhartelemy Varela in Friday's semi-finals, the Cuban having
outscored South Korea's Hong Moo Won.
Russia's Sergey Kazakov beat Namibia's Joseph Jermi 18-11 and fights Turkey's Atagun Yalcinkaya in the last four.
Zou defeated Bhartelemy Varela at last year's world championships in Bangkok and went on to win a silver medal.
Yalcinkaya outpointed Italy's Alfonso Pinto 33-24.