Djokovic through to semi-finals after downing Del Potro LONDON (Reuters) - Novak Djokovic joined Rafa Nadal as a guaranteed semi-finalist at the ATP World Tour Finals with a 6-3 3-6 6-3 victory over Argentine Juan Martin del Potro on Thursday. The 26-year-old Serb absorbed some brutish attacks from Del Potro before stretching away in the deciding set to earn himself a second victory in Group B. Del Potro now faces a shoot-out with Roger Federer on Saturday to see which of them will join defending champion Djokovic in the last four of the year-ender. Earlier Federer beat Richard Gasquet in straight sets to get off the mark after losing his opening match to Djokovic. Nadal is already assured of a semi-final berth having won both his Group A matches, sealing the year-end world number one ranking in the process.