The Princess of Wales took on Roger Federer in a game of doubles at Wimbledon, as the pair united to meet ball boys and girls before this year's Championships open on 3 July.They 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 Wimbledon.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."