The new Wilson Champions Choice Duo String blends natural gut with Luxilon ALU Power Rough for controllable power. Natural Gut 21ft6.7mLuxilon Alu Power Rough 20ft6.1m Natural Gut 21ft6.7m Luxilon Alu Power Rough 20ft6.1m. The string chosen by Roger Federer, the Champion’s Choice Duo combines the power and feel of Wilson’s Natural Gut with the durability and spin of Luxilon’s ALU Power Rough. Federer uses Natural Gut for his mains and ALU Power Rough for his crosses, for a softer feel and exceptional playability. For improved durability, use ALU Power Rough for your mains.