The Noam Elimelech (פרשת ויחי) says that before someone can take part in the high service of deepunifications, the yichudim, one must first struggle diligently to clear one's mind of all unrelated thoughts. All the thoughts of things pertaining to this world and not to more lofty purposes. This is a very difficult task and takes a long time to master. Only once one has cleansed themselves of these extraneous thoughts can one properly attend to the matters of yichudim. Know though, the Noam Elimelech goes on to say, even once a person has attained this high level, he will look back on his earlier work of clearing out his mind and recognize that it was/is the serious and real labor in avodat HaShem. He even goes on to say that the last seventeen years of Yaacov Avinu's life was involved in the basic labor of purifying his thoughts and that he valued that as highly as the rest of his life which had been involved in lofty unifications.
From here we can take a step to segue momentarily in the direction of today's Tanya where the Baal HaTanya brings down that אהבת עולם is the love one develops towards HaShem by finding the Godliness in the wordly things around him. This level of love is achieved by divesting oneself of the value of anything worldly and ultimately connecting to Godliness directly.
Then we can walk back to the Notzer Hesed (end of Avot:3) where he brings down that Yichudim (specifically) and tefillah (more generally) are called דרך ארץ (derech eretz) as opposed to Torah. So the mishnah points out that without derech eretz there can be no Torah and vice versa. So, Yichudim are half of man's work in the world. The other half being Torah study. (In the past I've brought down quotes from the Notzer Hesed about Yichudim being called the life of the moment, and Torah, the life of the world.)
But I think the Noam Elimelech gave us the clearest case about where Yichudim fall in the realm of avodah, service of HaShem.