But at Wimbledon, Her Royal Highness The Duchess of Cambridge reportedly revealed that the Swiss has a special five-year-old fan: Prince George."That's very big", Federer said, cracking a smile after winning his second-round win at The Championships.
Keothavong said the duchess had also been "very impressed" by 15-year-old Cori Gauff's remarkable win over Venus Williams on Monday.
During royal appearances, the Duchess of Cambridge regularly gets involved with sporting activities such as soccer, hockey and netball. The custom-designed Suzannah dress, which featured black buttons and a bow on the lapel, was cinched with a bow belt from Alexander McQueen, which the 37-year-old has previously worn.
"The whole family loves tennis, and George is apparently quite a good player".
"I sensed he was a bit nervous in the beginning with his double-faults", said Federer.
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"She s an incredible little girl who gives me so much inspiration".
Federer, who is chasing a 21st major title and ninth at the grasscourt major, is not the only competitor at Wimbledon with royal friends. It's such a quintessential part of the English summer, and I think it really inspires youngsters, myself, it inspired me, when I was younger to get involved in the game.
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Speaking about "cute" George's early tennis skills, Federer said: "At that stage it's all about just touch the ball, it's already good".
The duchess is expected to take her seat in the royal box later on Tuesday, which usually features famous faces and celebrity fans of the sport.
Kate made the rare move of watching some of the tennis action on one of the All England Club's outside courts, watching the first set of Dart's first round match.