By the time Roger Federer steps onto a tennis court, it's already been a long day for Ron Yu. For the last 15 years, in non-descript hotel rooms around the world, Yu has personally strung Federer's rackets -- nine fresh ones per match. Most pros just use local stringers provided by the tournament, and that was good enough for the players who made this year's U.S. Open finals. But for a hefty fee, some elite players hire one stringer to follow them around the world. Brook Silva-Braga reports.