Anthony Joshua
Joshua was one of the most marketable heavyweight boxers on the planet. However, he made a terrible error when he agreed to fight Andy Ruiz on short notice. Ruiz replaced Jarrell Miller after the latter failed a drug test. Nobody gave the heavyset Mexican fighter a chance because of his rotund physique.
Later it emerged that Joshua suffered a recent knockout in training. Perhaps this affected his performance because he failed to overwhelm his opponent. He knocked Ruiz down early but the Mexican came storming back. Then Ruiz secured the finish after Joshua failed to answer the referee’s count in the seventh round (via Insider).