Juan Martin Del Potro
It’s rare for a former U.S. Open champ to come into the event as a dark horse, yet such is the fate of del Potro. In 2009 the then 20-year-old Argentine blazed through the draw, taking down the mighty Nadal and Federer in succession and winning the first major of his career. Then, as if it were a Mephistophelian bargain, he promptly injured his wrist and missed virtually all of 2010. Having started this year ranked outside the top 250, del Potro is healthy again—clubbing forehands, winning matches, and fighting his way back to where he once belonged.