The 29-year-old Australian lost by split decision to Sebastian Fundora (21-1-1, 13 KOs) and forfeited the title. The American took the fight on two weeks notice, replacing injured compatriot Keith Thurman.
"It didn't make a difference. I was able to adapt in the first two rounds, it was comfortable, but he was able to win. We have what we have. He was the better fighter. He won by a one-round advantage, split decision on judges' scorecards, what we have is what we have. We move on," Tszyu said at the press conference.