At my last startup, one of our employees approached this well. This is a very summarized version, but he came to me and said: "I'd like to be paid $X. I think I deserve it and I believe I can get that salary on the open market. I don't want to negotiate on this."
It was a very significant raise but after a short discussion with my cofounders, we agreed and gave him what he asked for. That he was a top performer made it an easy decision.