About the Djokovic thing, this should be a void. If he doesnt participate its a void, thats how its supposed to be.
In the beginning I thought the same and have had a lot of discussions with multiple casinos around this. When placing an outright bet (like who will win the US Open), it doesn't matter if the player starts the tournament or not. I am pretty sure that all bookies will see this as a loss.
I think it varies by book for futures.