There’s a lovely sequence in the second episode of this four-part documentary about the career of Spain’s greatest ever tennis player. It’s 2007 and Roger Federer and Rafa Nadal are walking on to Wimbledon’s Centre Court to play the first of the many finals they would contest. Federer is poised and slightly smug; hair flopping perfectly over his headband, dressed in an immaculate white blazer. Nadal trails behind him, wearing a vest and baggy shorts, shaggy hair flowing and eyes wild, looking for all the world like a beautiful young caveman. It captures his initial appeal perfectly: in his early years, Nadal was elemental, athletic beyond description and impossibly charismatic: equal parts tennis player, action hero and acrobat. It feels like our sporting legends are increasingly reluctant to leave the stage. Lionel Messi (38) and Cristiano Ronaldo (41) will both be at this summer’s football World Cup. One of England’s greatest ever cricketers, James Anderson, turns 44 this year and is still plying his trade in the County Championship. Becoming unsurpassably brilliant at something requires …