Novak Djokovic defeats Stan Wawrinka in straight sets . Djokovic can secure end of year World Group spot with win in final group match . Tomas Beck overcomes Marin Cilic in straight sets . Roger Federer faces Andy Murray on Thursday .
Novak Djokovic defeats Stan Wawrinka in straight sets . Djokovic can secure end of year World No.1 spot with victory in final group match . Tomas Berdych overcomes Marin Cilic in straight sets . Roger Federer faces Andy Murray on Thursday .