LONDON (Reuters) - World number one Novak Djokovic had enough on his plate already on Wednesday without marauding Russian Daniil Medvedev pummelling him to a shock 6-3 6-3 defeat at the ATP Finals.Shortly after news broke that Djokovic had been nominated for a return to the ATP player council he had quit in August, he took to court at the O2 Arena hoping to seal his passage into the semi-finals of the prestigious year-ender. Instead his progress out of the Tokyo Group looks tricky, even if not quite as complicated as the politics rumbling on in men’s tennis since he announced in August he was forming a breakaway Professional Tennis Players’ Association (PTPA). Medvedev’s sensational performance means he is guaranteed top spot in the group and Djokovic, bidding to match Roger Federer by winning the title for a record-equalling sixth time, must beat 2018 champion Alexander Zverev in his last round-robin match on Friday. There was little sign of what was to come in a mesmerising opening to the match as Djokovic edged into a 3-2 lead. But the 24-year-old Medvedev, his flat groundstrokes scorching through the court, found a gear that even indoor master Djokovic could not locate.