After 5-hr quarter, Nadal retires from US Open semi after 2 sets
World number one Rafael Nadal crashed out of US Open by retiring from his semi-final against Juan MartÃ­n del Potro due to knee injury on Friday. Nadal, who had won his quarter-final in nearly five hours, quit when the Argentine was leading 7-6(7-3), 6-2. "It was difficult for me to keep playing. I was in too much pain," Nadal said.