Two-time Olympic gold medallist Zou was a strong favourite to beat Sho Kimura on Friday, but he ran out of juice as the fight wore on and the younger Japanese silenced a rowdy home crowd with an 11th-round upset victory.
Zou, whose professional record now stands 9-2 (two KOs), might be in the twilight of his career at 36 but he still wants his belt back.
"I believe in fate, but am not resigned to it. I will go on fighting until my last breath," he posted early on Monday on Weibo, China's equivalent of Twitter.
"A man must keep fighting to recover what he has lost."
Zou won the vacant World Boxing Organization (WBO) flyweight belt in November in Las Vegas, but since turning professional in 2013 he has largely failed to replicate the dominant form of a hugely successful amateur career that included two Olympic golds and three world titles.
Zou was too clever for the less experienced Kimura, 28, in the early rounds of Friday's bout but the Japanese -- a part-time restaurant worker -- kept coming forward and refused to be put off by a nasty wound he sustained above one eye.
Kimura, who improved to 15-1-2 (eight KOs), unleashed a flurry of punches in the 11th that stunned Zou, sending him to the canvas.
Zou, who was promoting himself for the first time after parting with US promoters Top Rank, was left in tears in the ring.
His wife Ran Yingying, a former television presenter who is heavily involved in her husband's new promotion company, sent a similarly defiant message in the early hours Monday on Weibo.
"At dawn I heard a steadfast pledge. I also want to say: I believe in you, and I know you.
"Restore what has been lost! A sincere man keeps fighting!"
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