Complex sentence: His stint as world champion did not last long, however, and he lost the title during his first defense, 370 days after winning it, being knocked out in eleven rounds by In Chul Baek, also in South Korea.
Split into simpler sentences: His stint as world champion did not last long, however, and he was defeated by while trying to defend the title for the first time. He lost the championship during his first defense, 370 days after winning the title, being knocked out in eleven rounds by In Chul Baek, also in South Korea.