If I control a Battlefield and have Glass Mixologist and Xin Zhao at the same Battlefield die by combat, can I revive that same Xin Zhao to that same Battlefield?

Yes, you can revive that same Xin Zhao to that same battlefield.

According to RiftJudge FAQ #9620 and FAQ #8739, when your Glasc Mixologist dies during combat, its Deathknell ability triggers and is placed on the chain as a pending item during the cleanup process. Because the game pauses to resolve this chain before finalizing the combat result and control checks, you are permitted to play a unit from your trash to that battlefield.

Sequence of Events

1. Combat Damage Step: Both Glasc Mixologist and Xin Zhao take lethal damage and die.
2. Deathknell Trigger: Glasc Mixologist's Deathknell triggers and is placed on the chain.
3. Resolution: The chain resolves. You resolve the Deathknell effect, which allows you to play a unit with cost no more than [3] and no more than [A] from your trash.
4. Unit Entry: You choose Xin Zhao (Energy cost 3) from your trash and play it to the battlefield where it just died.
5. Combat Cleanup: The game proceeds to finish the combat cleanup. Because you now have a unit present at the battlefield, the battlefield will not become uncontrolled, and the "no result" or control determination will factor in your newly revived Xin Zhao.

Note: This is based on the mechanics of Deathknell and battlefield control as established in the provided RiftJudge FAQs (specifically #9620 and #8739).