Roger Federer is back to his peak ... time to have a little off-court fun by surprising the ball boy who picked up his 10,000 ace ball. Federer next faces Tomas Berdych in the semifinals.

In a new video, the Princess of Wales, 41, gets the chance to challenge tennis champion Roger Federer on the court, and also gets a behind-the-scenes look at what the Ball Boys and Girls at ...