The x is Pacific's logo. Youzhny's string of choice? The USRA Survey top-rated Pacific Tournament Pro ToughGut, a poly/gut hybrid string that offers gut's love, power and control plus durability, $22.95 per set. Not cheap but anything involving gut never is.
Youzhny's racquet is the Head YOUTEK Extreme Pro, MSRP $190. You will usually see both the Head and Pacific logos painted on the strings. His apparel? adidas.