10-1 is not good enough up 3-1.
15-1 is realistic.
Just for the sake of discussion, down 3-0 I'd say he woulda been +500 to win the 3rd set and then +120 to win the 4th (if he had won the 3rd). Parlayed together that's about 12/1 which I think was probably the fair price.
Whole different ball game now though if Joker breaks back of course.