Novak Djokovic sent out a warning to his rivals at the ATP Finals on Monday, brushing aside big-serving John Isner 6-4, 6-3 to launch his bid for a record-equalling sixth ATP Finals title in style.

World No. 1 Djokovic, though, was playing a different game, appearing little troubled by the howitzers coming off the giant Isner's racquet — breaking his opponent three times and not conceding a single break point on his own serve.