Raducanu beat the 19-year-old Canadian Leylah Fernandez 6-4, 6-3, making her the youngest Grand Slam singles champion since 17-year-old Maria Sharapova won Wimbledon in 2004.
She is also the first woman ever to win the title in only her second Grand Slam tournament.
Emma Raducanu celebrates with the trophy after winning the 2021 US Open Tennis tournament. AFP
Following her win, Kate, who is known to be an avid tennis fan having frequented Wimbledon several times over the years, tweeted her congratulations via the Kensington Royal Twitter account, signing off with her initial “C”.
“Huge congratulations @EmmaRaducanu on your stunning performances and historic Grand Slam victory!” she began before going on to congratulate Fernandez.
“Incredible –we are all so proud of you. @LeylahFernandez well done on your amazing achievements at this year’s #USOpen, it’s been a pleasure to watch. C.”
But Kate is not the only royal family member to offer her congratulations.
The Queen also sent the US Open champion a message, hailing her “remarkable achievement.”
“I send my congratulations to you on your success in winning the United States Open Tennis Championships,” a message from HRH to the teenager read.
“It is a remarkable achievement at such a young age, and is testament to your hard work and dedication.
“I have no doubt your outstanding performance, and that of your opponent Leylah Fernandez, will inspire the next generation of tennis players.
“I send my warmest good wishes to you and your many supporters.”