Much as I'd love to see Zhang do it, I just can't see him holding Joyce off who has a mighty punch of his own.
I favour Joyce to wear him down and stop him at some point during the second half of the fight, but make no mistake he will need to use his slickness in this one and not leave his chin out to dry like he has been guilty of in numerous fights. I don't think he will but if he does Zhang is one of the few HWs who is capable of cracking that iron chin of his.
The fact he's a counter puncher is a concern for me. It's one thing to take a big shot you know is coming and are braced or prepared for it's another to take a clean shot when you don't expect it or see it coming. Even if Dubois hits harder than Zhang, I suspect Zhang to be a much bigger threat of hurting Joyce. Joyce may very well have a very tough time early on if he tries to press too much, he needs to be patient and cautious and take his time.
Personally I think Joe will need to be more worried about Zhang's sneaky right hook than straight left. So yeah, Zhang has a chance at pulling the upset. Although I think Joe will realize that and play it safe early like he did against Dubois, wear him down until he gasses and then mangle him.
KO Joyce? Nahhh ... Zhang is good and will light up Joyce early in the fight, that defence is not much. But the incredible engine and chin of Joyce will prevail in the second half of the bout Great and fun matchup