Need a new titanium racket or just some tennis lessons? How 'bout a new ball machine, to rent or own? Well, then, log off Anna Kournikova's Web site and head down to Game-Set-Match, near the Park Meadows Mall, for tennis-knowledgeable staff -- they'll even keep track of the tension at which you set your strings -- and high-end merchandise you might not find anywhere else. For instance: Want to know why you should play high-altitude balls in Denver? The answer, according to a ball salesman, is that high-altitude balls are manufactured at a lower air pressure than regular balls. This compensates for the lower air pressure around us, and slows down the balls a bit. Balls at altitude move faster and bounce higher than normal.