Light-welterweight Tom Stalker is the sixth British male boxer through to the Olympic quarter-finals after beating India's Manoj Kumar in his opening bout.
Stalker, who received a first-round bye, showed signs of ring rust in a torrid encounter.
After a fast start, the GB team captain tired alarmingly but held on to clinch the decision 20-16.
of the light-flyweight contest.
Stalker, 28, beat Kumar in the quarter-finals of last year's World Championships.
And the Liverpudlian, obviously fired up having spent the last week on the sidelines, came charging out of the blocks, taking the first round 7-4.
However, the second round could have gone either way, with Stalker playing into Kumar's hands and the Indian having plenty of success from close range.
But Stalker carried a seven-point lead into the final round and, although he looked very weary indeed when the final bell sounded, he held on for the win.
"I can improve a lot," he said. "I felt like I was staying there and fighting too much, rather than hitting and moving. My next bout I'll be a lot better. If I want to win the gold medal there's got to be a lot more to come.
"After having over a week in the village I didn't feel 100% sharp. But the fans got me through it - I'd like to thank every single one of them for such amazing support."
Stalker will fight Mongolia's Munkh-Erdene Uranchimeg for a medal after the world number two saw off Richarno Colin of Mauritius 15-12.
On the opposite side of the draw to Stalker, Brazilian world champion Everton dos Santos Lopes was beaten by Beijing bronze medallist Roniel Iglesias Sotolongo of Cuba 18-15.
Barnes, a bronze medallist in Beijing, picked off Thomas Essomba to secure a comfortable 15-10 win, before tweeting news of his victory from the ring.
The 25-year-old said: "Job done from the Olympic ring. Paddy Barnes - bringing you closer to the action." He later added: "Think I'm the first boxer to tweet from inside [the] ring."
Next up for Barnes, who is the third Irishman into the quarter-finals, is India's Devendro Singh Laishram, who beat fourth seed Serdamba Purevdorj 16-11.
The man who beat Barnes in Beijing, China's three-time world champion Zou Shiming, beat Yosbany Veitia Soto of Cuba 14-11.