Kate Middleton has joined eight-time Wimbledon champion Roger Federer for a behind-the-scenes look at the work of the “amazingly professional” ball boys and girls who are a key part of the tennis tournament – which takes place next month.The Princess of Wales and Roger, both 41, were guests at a training session for some of the young hopefuls aiming to be chosen for the around 250 ball boy and girl roles during the championships running from July 3-16 in south-west London. Kate, who is patron of The All England Lawn Tennis and Croquet Club which hosts the Grand Slam event, said: “Wimbledon is renowned for its amazingly professional ball boys and ball girls, the amount of work it takes, it’s incredible to see it behind the scenes.