Kate joined a homecoming celebration, hosted by the Lawn Tennis Association (LTA) at the National Tennis Centre in London on Friday.
Avid tennis fan Kate got to congratulate 18-year-old Emma, Joe Salisbury, Alfie Hewitt and Gordon Reid in person at the event and heard about their experiences of the tournament in New York.
The Duchess, who has been patron of the LTA since 2017, and is a regular at Wimbledon.
Following Emma's victory against Canada's Leylah Fernandez at the US Open final on 11 September, Kate shared a personal message on Twitter, writing: "Huge congratulations @EmmaRaducanu on your stunning performances and historic Grand Slam victory!
"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."
WATCH: Kate Middleton hits the tennis court with US Open champion, Emma Raducanu
The Duchess was dressed to hit the court with Emma and Gordon Reid, who won silver and bronze medals at the Tokyo Paralympics before he and doubles partner Alfie Hewitt won the US Open.
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The Duchess was in her element as she played doubles with the US Open champion. Kate reportedly used to take tennis lessons at the exclusive Hurlingham Club in Fulham.
Afterwards during an online BBC show, Emma was asked if she had more nerves playing Kate or her US Open finals opponent Leylah Fernandez, and replied: "I was actually very nervous playing the Duchess – I was like: 'Don't miss, don't miss'."
The 18-year-old said of Kate's tennis skills: "Her forehand was incredible. I was very impressed. It was a lot of fun to play tennis with the Duchess."
The Duchess beamed as she posed for a group photo with Emma Raducanu, Joe Salisbury, Alfie Hewett and Gordon Reid.
Before the group picked up their racquets, Kate told the champions: "Amazing guys, honestly, congratulations to all of you – it's seriously impressive."
What a shot! It's been a busy week for Kate, who tried out mountain biking and abseiling with a group of Air Cadets in Cumbria on Tuesday.
The Duchess also hosted a reception at St James's Palace to thank partners of her photographic community project, Hold Still. Kate wowed in a blazer-style dress with a pleated skirt from Self Portrait.
Queens of the court! There were plenty of giggles from Kate and Emma during their match.
Kate wore a navy tracksuit jacket and a tennis skirt, both by Poivre Blanc, for her sporty engagement.
The Duchess also donned white tennis shoes and tied her hair back in a ponytail, accessorising with tiny gold hoop earrings.
At one point, the Duchess was pictured celebrating a shot!
Kate chatting with Emma and Joe Salisbury, who won the men's doubles title with American Rajeev Ram at the US Open.
Joe said of meeting the Duchess: "It was really good fun. I was very impressed by Kate's level. She obviously loves it, and has played a lot before, but it was good fun to be on court with her. Her forehand was very good."
The Duchess also met teenagers from across the UK who are playing tennis at a grassroots level as part of the LTA Youth programme and hear more about the work that the LTA is carrying out to encourage participation in the sport amongst young people across the country.
Kensington Palace says Kate is passionate about supporting grassroots tennis and encouraging young people from all backgrounds to become involved in the sport.
LTA Youth is an innovative programme aimed at inspiring the next generation of players aged 4-18. The programme aims to help more children enjoy the benefits of playing and staying in tennis, whatever their age, gender, ability, disability or background.