The 37-year-old Duchess sat beside the Duke of Cambridge who looked dapper in a grey day suit and black sunglasses.
Kate, who is a patron of the All England Lawn Tennis and Croquet Club, pinned a purple and green checked bow to the left breast of her dress to symbolise her role in the organisation, Town and Country Magazine reports.
The future King and Queen consort of England are avid tennis fans, with George and Charlotte currently enrolled in lessons at Hurlingham Club in London, Hello! Magazine reports.
The Cambridges have even installed a new tennis court at their country home of Anmer Hall in Norfolk, according to Hello!.
On Saturday, royal fans were shocked when Kate made a surprise appearance with her sister-in-law Meghan Markle to watch Serena Williams play in the ladies' singles final at Wimbledon.
All eyes were on the Duchess of Cambridge and the Duchess of Sussex as they sat side by side along with Kate's sister Pippa Middleton, in an apparent attempt to quash rumours of a royal rift once and for all.
WATCH Kate Middleton and Meghan Markle put feud rumours to bed by sharing a laugh at Wimbledon:
Kate stunned in a fitted green dress from Dolce and Gabbana, while new mum Meghan - who welcomed her and Prince Harry's first child baby Archie Harrison on May 6 - looked sophisticated in a graphic printed skirt by Hugo Boss and a crisp white shirt from Givenchy.