Mark Murphy's music is not universally respected. In fact, I can think of no other singer so influential who is disliked by so many critics. Should this bother you? Probably not; it certainly doesn't bother Murphy, who continues to compel the rest of us with his refusal to relax, his abhorrence of complacency. His rich baritone has a distinctive authority that's rare in any idiom; what's more, he is one of the very few jazz singers who understand the real demands and possibilities of jazz improvisation. Even among jazz singers, the liberties Murphy takes are remarkable: holding a note way beyond its expected length and resolving the rhythmic tension with a breathtaking double-gainer; hovering between the patterns of speech and pure musicality; at times abandoning all pretense of language to lose himself in a swirl of inchoate vocalization. Murphy takes risks like nobody's business, and they don't always pan out. But even when some audacious melodic jump or brash line reading falls flat and causes me to wince, Murphy commands my respect: these events go with the territory, as Murphy continues to stretch his interpretive skills with each performance. And far more often than almost any other vocalist, his leaps of artistic faith open yet another window into the human condition. Tonight, 9 PM, and Saturday, 8 PM, Green Mill, 4802 N. Broadway; 878-5552.