If I use Atakan eff to reduce his cost sacrificing my only unit on battlefield, can I play Atakan on that battlefield or do I lose the control before it?

Yes, you can play Atakhan to the same battlefield.

According to Rule 187.4.c (as updated in the Unleashed rules), you do not lose control of a battlefield while there are items on the chain.

The Sequence of Resolution

1. Playing Atakhan: You begin the process of playing Atakhan. You choose to pay his additional cost by killing your only unit at the battlefield.
2. Chain State: Atakhan is placed on the chain as an item. Because an item is on the chain, the state is CLOSED.
3. No Immediate Cleanup: Although the battlefield now has no units on it, Rule 187.4.c specifies that you cannot lose control of the battlefield while there is an item on the chain.
4. Finalization: Once Atakhan resolves and enters play, your unit is now present at the battlefield. Because you have a unit there, you successfully maintain control of the battlefield.

As stated in the Rules FAQ Clarification (2026-04-29):

"Control of battlefields cannot be lost while there are items on the chain, per rule 187.4.c. This means that spells and abilities that... kill units as a cost or effect in order to play another unit, will be able to kill or banish units at battlefields and then play the resulting unit to that same battlefield."