Juan Martin Del Potro, the former US Open champion whose career has been damaged by the wrist injury which sidelined him in 2010, on Thursday revealed some of the obstacles to returning to his former prominence after reaching the semi-finals of the Dubai Open. Del Potro, who has won no further Grand Slams and just seven titles since the career-threatening setback, kept alive his chances of an eighth by beating Daniel Brands, a surprising German qualifier, by 6-4, 6-2.