Vasyl Lomachenko's attempt to claim the WBO featherweight title in only his second professional bout ended with defeat by Mexican Orlando Salido.
But he lost a split points decision against Salido, 33, in San Antonio.
Salido did not make the weight for the fight, so the title will become vacant.
He had already stated the bout would be his last as a featherweight.
Lomachenko was widely regarded as the best amateur boxer of all time, having built an alleged 396-1 record.
He went into the Salido fight in confident mood.
"My argument is two gold medals from the Olympics, two gold medals from the World Amateur Championships and my first professional bout was a knockout with a guy who was in the top 10," said the Ukrainian.