Whenever the phrase "tenor battle" (or the more imaginative "Battle of the Saxes") pops up, modern musicians rush to point out that such events are misnamed--that contrary to the old days, when cutting contests actually did find instrumentalists pitted against each other in a fight for dominance, today's players have moved beyond such considerations and engage in far friendlier collaborations. But Chris Potter and Evanston's Eric Alexander slugged it out for real in the fall of 1991, when Potter finished third and Alexander second in the Thelonious Monk International Jazz Competition for tenor saxists. I'd hate a choose between them, for the same reason I can't wait to hear them together: the somewhat flashier Alexander and the slightly more focused Potter complement each other in much the some way as Al Cohn and Zoot Sims did in the most famous of jazz's two-tenor partnerships. Potter has the edge in fame, having toured and recorded with trumpeter Red Rodney's band; Alexander, recently repatriated to New York, has two of his own albums awaiting release, not to mention the hometown rooting section. But no matter whether this "competition" encompasses a true rivalry: it brings two of the top young tenors to the same stage, and that's promise enough. Friday, 9 PM, and Saturday, 8 PM, Green Mill, 4802 N. Broadway; 878-5552.