Kierkegaard and Nietzsche are often treated together as early existentialists. This is rather odd, as Kierkegaard was self-consciously seeking to be an orthodox Christian, and Nietzsche was self-consciously seeking to destroy Christianity.
Nevertheless, after reading only a little bit of Nietzsche I have seen some parallels to Kierkegaard already. (Citations below from The Portable Nietzsche). For example:
Your self itself wants to die and turns away from life. —Thus Spoke Zarathustra, Part I, "On the Despisers of the Body", 147
This bears similarity to Kierkegaard's idea of "in despair not wanting to be oneself" (Anti-Climacus in The Sickness Unto Death). Also:
The actor has spirit but little conscience of the spirit. —Thus Spoke Zarathustra, Part I, "On the Flies of the Market Place", 164
Once again, Kierkegaard speaks much of spirit and consciousness of spirit/self.
Wikipedia claims that Nietzsche said he would read Kierkegaard. Do we know whether he actually did? If not, can their commonalities be traced back to some other philosopher they both read? For example, perhaps "consciousness of spirit" is terminology from Hegel?