When an opponent contests my battlefield and kills my only unit there with Hextech Ray, can I play Sprite Call to that battlefield during the contest showdown?

Ruling: You cannot play units to a battlefield you don't control. If your opponent kills your only unit at a battlefield, you immediately lose control and cannot play units there.

Nuances:

• If you still have other units at the battlefield when your opponent kills one unit, you can play the sprite there during the showdown since you still control it
• If you activate Hidden Sprite Call as a reaction to Hextech Ray (before your unit is killed), you can play the token to that battlefield since your unit has not been killed yet