We are not serving Hashem Himself - His עצמיות. We are serving His NAME - שמו [that is the הכרעה of the בעל הלשם]. The שם is Hashem's עצמיות as WE PERCEIVE IT. The "Name" of Hashem preceded this world, as it says in פרקי דרבי אליעזר :
"עד שלא נברא העולם היה הקב"ה הוא ושמו אחד"
Before the world was created Hashem and His Name were one.
Our Avoda of His Name is explicit in various psukim and in the Mishna:
"ליראה את השם הנכבד והנורא" [דברים כ"ח נ"ח]
"ויברכו שם כבודך" [נחמיה ט' ה']
"בכל מקום מוקטר מוגש לשמי" [מלאכי א' י"א]
"לשם ששה דברים הזבח נזבח, לשם השם" [זבחים פ' ד' משנה ז']
We aren't serving his מידות [as the Nefesh Hachaim says ש"ב פ"ד בהגה"ה] such as חסד or גבורה.
We are serving His שם and הוא ושמו אחד.
