Frazer Clarke defeated Tsotne Rogava with power and precise punches as Team GB's super-heavyweight ended a wait of over 10 years for his Olympic debut.
The 29-year-old had patiently prepared for his opening Olympic appearance and he marked it successfully by sealing a 4-1 points victory over his Ukrainian opponent.
Clarke claimed the gold medal at the 2018 Commonwealth Games and is attempting to emulate the Olympic achievement of Anthony Joshua, who won the super-heavyweight tournament at London 2012.
He had previously told Sky Sports: "I'm a smart boxer, I am experienced.
"I probably need a better defence! I'm always in exciting fights, people tell me I am exciting for one reason or another!
"I dish it out, I've got fast hands, good thinking, good feet, I'm strong, I can really box. When it's time to grit and bite down on the gumshield? I can do that as well."
But Clarke will need to overcome a dangerous crop of rivals, which contains No 1 seed Bakhodir Jalolov, who ominously advanced in his first bout.
The Uzbekistan fighter, who has eight victories as a professional, forced Azerbaijan's Mahammad Abdullayev to receive a standing count during a dominant points win.
Davison denied in last-16
Charley Davison hopes of securing a medal were ended by a 5-0 defeat to No 2 seed Chang Yuan in the last-16.
The 27-year-old had produced a dominant win over Rabab Cheddar in her opening bout, but Davison could not contain Yuan, who sealed her unanimous victory with crisp counter punches.