Can Player 1 flip a hidden Zhonya's after damage has been assigned but before damage is dealt during combat?

Ruling: No, once you have moved to assigning damage there is no chance to react. Your last chance to flip Zhonya's is during the showdown phase before damage assignment.

Sequence:

• Last opportunity to flip Zhonya's is during showdown (before damage assignment)
• Once damage assignment begins, no actions or reactions can be taken
• Damage is dealt with no opportunity to respond

Nuances:

• If a unit with Deathknell dies in combat, this creates a chain after combat cleanup where you could flip Zhonya's to save it from being discarded (since hidden cards not belonging to the battlefield controller are discarded during cleanup), but this will NOT save any units from combat damage as they are already dead
• The exact timing of when contested status is removed relative to Deathknell triggers is not officially confirmed in the current rules, though community consensus is that Deathknell creates a chain before contested is removed
• This Deathknell timing interaction is a known rules gap awaiting official clarification