Defending champion Novak Djokovic trounced stand-in David Goffin 6-1 6-2 for a clean sweep of his group at the ATP World Tour Finals on Thursday but spiced up a leisurely afternoon workout by picking a row with the umpire. The world number two, who will be joined in the last four by Milos Raonic after the Canadian beat Austrian Dominic Thiem 7-6(5) 6-3, began the week at London's O2 Arena quarrelling with a reporter and despite being relatively untroubled on court, there has been an edginess about the 12-times grand slam champion.