Cuba's 100% record in the ring has ended after Yoan Hernandez was out-pointed by Russia's Evgeny Makarenko in their light heavyweight contest.
Makarenko made height advantage count, pummelling his opponent to finish with a convincing 30-18 victory.
"It wasn't that easy - beating a Cuban boxer is always an achievement," said a delighted Makarenko.
The Russian, twice a world and European champion, now faces American Andre Ward for a place in the semi-final.
Ward, who has not lost a decision in six years, out-pointed Italian Clemente Russo 17-9.
"I had butterflies in my stomach and I couldn't wait to get started," said Ward.
"I didn't see the Makarenko bout. I know he's a very good boxer but I'm determined to win gold, so I guess I have to beat him."
In other action, Cuba's Lorenzo Aragon, a 30-year-old in his third Olympics, saw off inexperienced American Vanes Martirosyan to win a 20-11 decision at 69 kg.
Also at 69kg, two-time Olympic gold medalist Oleg Saitov of Russia advanced to the quarter-finals with a narrow 18-17 win over Mohamed Hikal of Egypt.
And Greek light heavyweight Elias Pavlidis moved to within one win of a medal with a 31-16 victory over Ali Ismayilov of Azerbaijan.
Elegant Frenchman Willy Blain beat the hard-hitting Russian Alexander Maletin 28-20 in an exciting clash of styles.