Actually Louis was a 15-1 favorite in his bout with Lewis, BUT that wasn't the record. Against Harry Thomas, Al McCoy, Gus Dorazio and Tony Musto, he was a 20-1 favorite. He was also a 18-1 favorite against Abe Simon the 1st time they fought.
7-1 was the price on Lewis, not on Louis. But really I know nothing about gambling. Odd that Simon was longer the first time than the second time. I guess he did better than expected.
Louis was a 15-1 favorite over Lewis when they fought. By John Henry's own admission, he entered the ring against Joe Louis with his eyesight already seriously imaired. He was the undisputed champion of the world in his weight class but had little rnoney, and was going blind. It is a fact that he and Joe were close personal friends; and ring legend has it that the stricken John Henry begged Louis for the title shot that would bring him one big, final payday. The match was made, and on January 25, 1939, a crowd of 17338 paid 102,015 at New York's Madison Square Garden to watch 15- 1 favorite Joe Louis defeat his friend in just two minutes, 29 seconds. John Henry Louis never fought again