NEW YORK – Normally so cool, so consistent, so in control of his emotions and his matches, Roger Federer let the U.S. Open championship slip from his grasp. Two points from victory against inexperienced, unheralded Juan Martin del Potro of Argentina, two points from a sixth consecutive title at Flushing Meadows and a record-extending 16th Grand Slam overall, Federer, quite simply, fell apart Monday.