On the eve of his 20th appearance at the U.S. Open later this month, 39-year-old Novak Djokovic has no intention of keeping tennis fans in suspense.“I have no date in my mind, to be honest, the expiry date of my career,” Djokovic says in the opening minutes of “The Wolf in Winter”, the documentary released Thursday on Prime Video.“I know a lot of people are speculating that, a lot of people are retiring me for many years. Why would you want to retire me? When I’m still playing and competing at the highest level?”Djokovic is the subject of the latest documentary exploring the career twilight of a tennis great, and the noise around the end of that career — and what makes for a good ending — has a familiar timbre.But it undercuts the project early on by planting a nagging question in viewers’ minds. If Djokovic isn’t struggling with his decision to walk away, what is that discussion for?In his previous two sports documentaries, “The Last Dance” and “André the Giant,” director Jason Hehir fixed his camera on two larger-than-life celebrities and reminded audiences that they never truly know their heroes, employing delightful interview techniques and archival footage to investigate the psyches and forgotten backstories of Michael Jordan and the pro wrestler André René Roussimoff.Hehir, alongside Connor Schnell’s Words + Pictures and Sadoux Productions, chose a different challenge for Djokovic: Following a sporting giant whose reputation is much more complicated — and whose story arc doesn’t yet have an ending.Messi and Djokovic: Two 39 year olds still owning their sportsLukas WeeseFor sports fans new to tennis, or documentary buffs who don’t watch sports, the 90-minute film is a glossily packaged, bird’s-eye view of a 24-time Grand Slam champion — an excellent Djokovic 101. But for anyone who’s even been loosely aware of tennis’ storylines and main characters over the past 20 years, the film doesn’t probe enough to reveal anything new about the champion, and the incomplete nature of Djokovic’s story makes for a frustrating lack of stakes. It never quite answers a central question: Why now?Netflix’s incisive Rafael Nadal docuseries, “Rafa”, premiered in May, years after the Spaniard retired. Roger Federer also waited until retirement to release his documentary about the final days of his pro career. Other tennis players who have released projects have done so much earlier in their careers.Djokovic’s late-career film — the winter of the title — offers a teasing ambiguity that it never quite fully mines. Without his final act in place, it feels like the first part of a series, something Djokovic himself echoed in an interview about the project last month.