I have Xin Zhao and Glasc Mixologist at a battlefield. Opponent plays Singularity, killing both. Can I use Mixologist's Deathknell to play the Xin Zhao that just died to that same battlefield?

Yes, you can use the Deathknell ability of Glasc Mixologist to play Xin Zhao from your trash to the battlefield where they both died.

The Ruling

According to RiftJudge FAQ #9329, when a unit with Deathknell dies, its ability triggers and is added to the chain as a pending item during the cleanup process (Rule 322.3). Because the game pauses to resolve this chain before finalizing the cleanup and control checks, you are permitted to play a unit to that battlefield. If you successfully play a unit there, you will have a unit present when the game checks for control, allowing you to maintain ownership of the battlefield.

Sequence of Events

1. Singularity is played and resolves, dealing 6 damage to both Glasc Mixologist (5 Might) and Xin Zhao (4 Might).
2. Both units take lethal damage and are marked for death.
3. The game enters the Cleanup phase.
4. Glasc Mixologist's Deathknell triggers and is added to the chain as a pending item (Rule 808.1.d.2).
5. Both units are moved to the trash.
6. The chain resolves. You resolve the Deathknell effect, allowing you to play a unit from your trash that meets the cost requirements (cost no more than 3 and no more than [A]).
7. You choose Xin Zhao (Energy cost 3) from your trash and play it to the battlefield.
8. The cleanup process continues, and the game checks for control of the battlefield. Because you now have Xin Zhao present, you maintain control of that battlefield.

Note: This is my interpretation based on the mechanics of Deathknell and battlefield control as established in the provided FAQs. No official FAQ exists for this specific combination of cards, but it follows the standard resolution of triggered abilities during the cleanup process.