American teenager Jose Ramirez scraped past Frenchman Rachid Azzedine to maintain the United States' fine start to the Olympic boxing competition with victory in his opening bout in the lightweight division.
The 19-year-old Ramirez shaded a fast, powerful contest against his more experienced opponent, who threw everything forward in the final round but found it was not quite enough as Ramirez edged home 21-20.
Ramirez, who will fight Uzbek Fazliddin Gaibnazarov in the last 16, said: "I was a little tense in my first Olympic bout but hopefully I can be me in my next fight. I am part of a young team and we are all very hungry."
Also at lightweight, Kazakhstan's fifth seed and world bronze medallist Gani Zhailauov was lucky to survive against Thailand's Saylom Ardee, coming from behind to pull out victory on countback after a 12-12 draw.
Puerto Rico's Panamerican youth champion Felix Verdejo Sanchez, age 19, beat Juan Huertas Garcia of Panama 11-5 with a patient display of counter-punching, while Liu Qiang of China had too much for Australian Luke Jackson, coasting to a 20-7 success.
At welterweight, Errol Spence was another American winner as he took advantage of a clumsy performance from Brazil's Myke Carvalho, who was penalised in the second round for persistent holding, to go through 16-10.
Alexis Vastine enjoyed a successful return to the Olympic ring with a 16-12 win over Germany's Patrick Wojcicki, four years after departing in a fury after a controversial defeat forced him to settle for bronze in Beijing.
Two late points penalties sent Vastine to defeat against eventual winner Felix Diaz of the Dominican Republic, but Vastine insisted that disappointment was a thing of the past.
Vastine said: "It was a bad memory for me but you can't stay in the past, you've got to look forward. I want to look forward and to go further than I did four years ago. I have made a good start and I am feeling confident."
Also at welterweight, there were wins for Mongolia's powerful Tuvshinbat Byamba, Canadian Custio Clayton and Australia's world number five Cameron Hammond, who had too much for Nigeria's Abdoulaye Hima, winning 13-6.