Meanwhile Meghan opted for a striking blue striped Ralph Lauren shirt and a wide-leg white pants, teamed with court shoes and a fedora.
The pair arrived for the resumption of the men's semi-final between Rafael Nadal and Novak Djokovic ahead of Williams' bid to win her eighth Wimbledon title. Middleton, also 36, rocked a black-and-white patterned dress.
It's well-known that the Duke and Duchess of Cambridge are tennis fans - they've been spotted at many a match over the years.
Kate and Meghan, who are conducting their first engagement together without their husbands, stopped and spoke with a number of ball boys and ball girls, as well as junior players before making their way across the walkway for lunch.
The Duchess of Sussex made her first Wimbledon appearance as a member of the royal family on Saturday looking absolutely stunning in an incredibly chic outfit, but did she break Wimbledon's Royal Box dress code? It's obviously disappointing but I can't be disappointed, I have so much to look forward to and I'm literally just getting started.
Tiger Woods, Lewis Hamilton and Anna Wintour were in Serena Williams' box for championship day, giving the seven-time Wimbledon champ an A-list cheering squad.
Kerber ultimately took the title this time, but Williams certainly had the support of her millions-strong fanbase.
Serena Williams had attended Meghan's wedding a couple of months back.
Williams used to be the highest ranking female player but her seed plummeted to 25th after she left the sport to have her baby, Alexis Olympia Ohanian Jr. "But to all the moms out there, I was playing for you today and I tried", she said, as the cameras turned to the Duchess of Cambridge and the Duchess of Sussex, who were moved by her words. Kerber defeated Williams 6-3, 6-3, to win the tournament.
Wimbledon finalist Serena Williams was one of the hundreds of guests invited to the ceremony in St George's Chapel as the tennis star has been friends with the former Suits actress for several years.