Kate and the eight-time Wimbledon champion watched a training session where youngsters were hoping to impress selectors and bag one of the 250 spots for the tournament next month.

The princess, who is a patron of the All England Lawn Tennis and Croquet Club, said Wimbledon was "renowned for its amazingly professional ball boys and ball girls" and it was "incredible to see it behind the scenes".

She told them: "With all the discipline and everything you've learnt, you'll be able to take it into so many other parts of your life - things like confidence, and that feeling of pride with being on any of the courts is such a big moment."

Federer said he was "really impressed at how much effort and training goes into being a ball kid during the championships".

"I used to be a ball kid in Basel when I was nine or 10 and, as I have always said, once a ball kid always a ball kid," he added.

During the event, Kate took part in a doubles rally with Federer on the No 3 Court before watching the children take part in training.

Around 1,000 children aged between 13 and 15 take part in sessions every year at the club's community centre in Raynes Park.

Those that make it to the famous tournament will have completed months of strict training, where they learn key skills including how to signal a ball change, rolling the balls up to the net, and the correct way to bounce the balls to the players.

There will be 170 newcomers to the role, while another 80 will be returning from previous years.

Sarah Goldson, manager of the ball boys and girls squad, said: "It was a pleasure to welcome Her Royal Highness and Roger Federer to Wimbledon to see and experience what it takes to be a BBG at Wimbledon.

"We're really proud of the young people who dedicate so much time and effort to their role which plays an integral part in delivering a successful championships."

This year's Wimbledon tournament gets under way on Monday 3 July.