Meghan and Kate take their front-row seats in the royal box (left).
Meghan, who married Prince Harry in a magical ceremony in Windsor in May, is a good friend of Williams, who gave birth to her daughter past year.
Just reaching the final was an incredible feat for the 36-year-old, who nearly died and was left bedridden for six weeks following the hard birth of daughter Alexis Olympia last year.
Catherine, Duchess of Cambridge and Meghan, Duchess of Sussex attend day twelve of the Wimbledon Lawn Tennis Championships at All England Lawn Tennis and Croquet Club on July 14, 2018 in London. I have so much to look forward to.
Williams said: "We've always had a wonderful friendship".
Serena famously attended Meghan and Harry's wedding in May, and the pair spent a Sunday together watching Prince Harry play in a charity polo match in Berkshire just two weekends ago.
Also supporting Williams on Centre Court are golfer Tiger Woods, Formula One driver Lewis Hamilton, Vogue magazine editor Anna Wintour and writer-director Shonda Rhimes.
Middleton, a royal patron of the All England Club, will also watch the men's final, where she'll be accompanied by Prince William.
The tennis star, who had previously remained coy about whether Meghan would be seen cheering her on, spoke out on Thursday after Kensington Palace confirmed the news.
New mum Miss Williams could not complete the mother of all victories just ten months after giving birth and was beaten in straight sets by 11th-seeded German Angelique Kerber.
"I'm sure you will have your next grand slam title".