Answer: You can't.
What you can do is to give a discount specifically for this client for one particular SKU (or any client and a couple of SKUs if desired).
Discounts are covered in this article.
Let's say you want to set price tier 1 for one of your customers but for one particular item you give them a $2 discount because they found a better price.
You can set a discount rule "$2 off" (or you can use %) and specify the discount for the product, as shown in the screenshot below.
Here, I set a discount rule for all customers with the tag, but you can also specify individual discounts for customers. Note that the price for this item is $10.
When I click the Apply discounts button, the price changes to $8.