Markle attended the event in 2016, around the time she was set up on a blind date with Prince Harry, while Middleton has regularly attended matches with both her sister, Pippa Middleton, and her husband, Prince William.
Her outfit choice is said to be a nod to the designer, as Ralph Lauren supply the clothing for the Championship's ball boys and girls, lines judges, and umpires.
Kate, 33, will also attend the gentleman's singles final on Sunday alongside Prince William.
Settle in for this one.
Before adding: 'We always have supported each other, just been there for each other through a lot.
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.More news: Britain's Prince Louis to be christened in private ceremony
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"She's officially No. 1", Williams said of Middleton after complementing the Duchess' ability to look great right after giving birth.
The new mum also attended Meghan and Prince Harry's royal wedding earlier this year, where she appeared to be in high spirits with her hubby Alexis Ohanian.
Serena Williams returns against Germany's Angelique Kerber on July 14, 2018.
Meghan paired a loose fitting, blue and white striped Ralph Lauren shirt with a pair of wide-legged, high-waisted white trousers and a white straw boater hat.
"Every year, for a couple of years she comes out to Wimbledon, has supported me". Now she's supporting me in a different role.