CBS has sold more than 90 percent of its available commercial inventory for its two weekend telecasts of the U.S. Open tennis championships. The competition begins Aug. 28, but CBS' coverage will begin Saturday, Sept. 2, and run through Labor Day, Sept. 4, concluding the following weekend. CBS Sports Sales President John Bogusz says ad sales are pacing slightly up on a dollar volume basis compared to last year, but would not give specifics on costs. The four major sponsors of the telecasts are back: American Express, Lexus, MassMutualFincancial Group and IBM. Among the new advertisers on board this year are GlaxoSmithKline, AT&T and Apple Computer Inc.