The 18-year-old made history on Thursday by gaining a place in the US Open final after her victorious match against Maria Sakkari in New York.
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Taking to Twitter the following morning, Kate released a rare personal message to the young tennis star. She tweeted: "What an incredible achievement at this year's #USOpen @EmmaRaducanu!"
The tweet was signed off "C", the initial of the Duchess' full name, Catherine. The message concluded: "We will all be rooting for you tomorrow. Wishing you the best of luck! C."
Kate, a keen tennis fan and patron of the All England Lawn Tennis Club, will be rooting for Emma during her bid to win the grand slam tournament on Saturday.
Emma Raducanu has made history
The teenager is the first British woman to reach the final at Flushing Meadows since Virginia Wade won the title 53 years ago.
Prime Minister Boris Johnson also joined in the praise of the young tennis player on Twitter. He tweeted: "Well done @EmmaRaducanu for a brilliant win at the #USOpen. The whole country will be cheering you on in the final."
Immediately after her win against Maria, tennis star Emma said: "Honestly, the time here in New York has gone so fast. I’ve taken care of each day. Three weeks later and I’m in the final – I can’t believe it.
Kate sent her best wishes to the teenage sensation
"Tim Henman is honestly such a big inspiration, helping me to treat one point at a time, you can’t get ahead of yourself. Today I wasn't thinking about anyone else except for myself!"
She added: "I want to thank my team, the LTA and everyone back home. I wouldn’t be here without their support. I've had unbelievable support in New York you’ve made me feel right at home. Playing under the lights on Arthur Ashe is one of the most iconic things in tennis. Is there any expectation? I’m a qualifier so on paper, there’s no expectation on me!"
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