Novak Djokovic has threatened to skip tournaments in the future including the ATP Finals at the end of the year. The 37-year-old is keen to keep his playing career going as long as possible, which will mean managing his workload in his advancing years. Djokovic has already taken something of a backseat in some competitions over the past few years, instead concentrating on the Grand Slam events. With the other tournaments not offering the same prestige as the Australian Open, French Open, Wimbledon and US Open, Djokovic has admitted he might stop playing some ATP events altogether. “Torino [the…