Selimov (left) had no answer once Lomachenko got going
Pre-tournament favourite Albert Selimov crashed out of the Olympic boxing event in the first round after losing on points to Ukraine's Vasyl Lomachenko.
Russia's Selimov, the world and European featherweight champion, lost out 14-7 on the judges' scorecards.
Selimov led 2-0 after a tactical first round but Lomachenko upped the tempo to move well clear of his opponent.
Meanwhile, Russia's Alexey Tishchenko, 2004 gold medallist, beat Tunisia's Saifeddine Nejmaoui at lightweight.
Tishchenko, 24, won gold at featherweight in Athens, but has now moved up the divisions.
"I tried not to spend too much energy and defended mostly," he said. "The other guy got too close and clinched too much. It was not a very beautiful bout and I didn't like it."
Cuban lightweight Yordenis Ugas and featherweight Idel Torriente made impressive starts with comprehensive defeats of Algeria's Hamza Kramou and Ghana's Prince Octopus Dzanie respectively.
"I'm glad to see that the Cuban boxing school is working perfectly, still efficient," said Cuban head coach Pedro Roque.
"These young boys are going to be the worthy successors of the historic Cuban champions."