Complex sentence: He began his career in 1969 and quickly established himself as a formidable puncher, before his made his first world heavyweight title challenge on September 1977 against Muhammad Ali his impressive record stood at 54-5-1 (52 KOs).
Split into simpler sentences: He began his career in 1969 and quickly established himself as a formidable puncher. Prior to his first world heavyweight title challenge on September 1977 against Muhammad Ali his impressive record stood at 54-5-1 (52 KOs).