Great Britain super-heavyweight Anthony Joshua survived a scare in his Olympic opener, pipping Cuban Erislandy Savon to reach the quarter-finals.
Savon repeatedly beat the Brit to the punch in the first round but lost it 6-5 and the pattern then repeated itself.
However, the judges appeared to favour Joshua's heavier punches over Savon's slickness.
Bantamweight Luke Campbell was the first GB boxer into the quarters with an 11-9 victory over Jahyn Parrinello.
Campbell and Joshua's victories mean all of Britain's seven male boxers remain in the competition.
And Irish bantamweight John Joe Nevin also won through courtesy of a classy display against Kazakhstan's Kanat Abutalipov.
The 22-year-old Joshua is seeded second at the tournament, while Savon, the nephew of three-time Olympic champion Felix, is ranked fourth in the world.
"It was a terrific fight and I think that was a gold medal round for sure. The way Joshua was throwing that right hand, those were knockout punches. He has a lot of raw talent, all we have to do is harness all that power and put it into some good boxing."
However, it was Savon, also 22, who looked the more accomplished fighter in the first, strafing Joshua with combinations.
Joshua began to impose himself more in the second, landing with some meaty shots, but Savon was still piercing his rival's defences, and got through with one jolting right hand in particular.
The Briton began to tire down the stretch as the more wiry Savon let his punches go, and the Londoner looked more than a little relieved when his arm was raised.
The next opponent for Joshua is China's Zhilei Zhang, who stopped Australia's Johan Linde in the second round.
"It's all about staying calm. Even though it's your home crowd you get in there and you zone in on something different," said Joshua.
"The atmosphere definitely spurred me on, but I was calmer than I thought in there.
"We were both whacking each other with shots, we kept fighting to the last bell and let the judges do the scoring.
"Savon is a tidy boxer, that being my first fight will put me in good stead for the rest. The atmosphere is unbelievable out there and I have to give them great respect."
World silver medallist Campbell is perhaps his country's best chance of gold in the boxing ring but he did not have things all his own way in his opening bout.
Campbell (left) admited he was nervous but believes he will be better for getting a fight under his belt
Having been handed a bye into the second round, Campbell, showing signs of nerves, was level at 3-3 after the first three minutes.
The 24-year-old, European champion in 2008, started finding his range in the second, landing with spearing jabs and flashing right hooks.
But Parrinello continued to make things messy until the final bell and Campbell was also relieved to have won through.
Campbell said: "I was nervous. I've had a little bit of a wait, an extra three or four days, and you can't do anything but think about it.
"And I've never seen a crowd like that in my life, it was overwhelming.
"I think I'll be better for that, calm down, settle into my rhythm and get better as the tournament goes on."
Next up for Campbell is Bulgarian Detelin Dalakliev, who he beat in the final of the European Championships in Liverpool and also at last year's Worlds.
"Let's hope I've got his number, because this is where it counts," said Campbell.
Nevin, who is in the opposite side of the draw to Campbell, gave a beautiful display of counter-punching to run out a 15-10 winner.
In Joshua's division, number one seed Medzhidov Magomedrasul, who beat the Briton in the final of the 2011 World Championships, stopped wildcard Meji Mwanba in the second round.
Italy's former world champion Roberto Cammarelle beat Ytalo Perea Castillo of Ecuador 18-10.
World bantamweight champion Lazaro Alvarez Estrada overcame a tough examination from American Joseph Diaz Jr, but came through 21-15.
Estrada, who beat Campbell in the final of last year's Worlds, was lucky to be ahead going into the last but had a strong final round.
Diaz Jr was one of three American boxers to lose on Wednesday, with heavyweight Michael Hunter and super-heavyweight Dominic Breazeale also bowing out.