The recently retired Andy Murray is going to team up with longtime rival Novak Djokovic as his coach, they both announced Saturday, with plans to prepare for — and work together through — the Australian Open in January. It was a stunning bit of news as tennis moves toward its offseason, a pairing of two of the most successful and popular players in the sport, both of whom are sometimes referred to as members of a so-called Big Four that also included Roger Federer and Rafael Nadal. Djokovic is a 24-time Grand Slam champion who has spent more weeks at No. 1 than any other player in tennis history.