It's the second visit to Wimbledon for both Catherine and Meghan this year after the Duchess of Cambridge watched Roger Federer at Centre Court last week and the Duchess of Sussex watched Williams in her second-round match against Kaja Juvan. "She couldn't be a better friend to me".
Meghan and Williams are close friends, with Williams attending her wedding to Harry last summer.
In a post-match press conference, Williams said that having Meghan's support and friendship "is great". "And always positive, no matter what", she remarked, adding that the royal is also "happy for Simona".
Hasan, who queued from 5 a.m. and paid £25 for his Court One seat, said he did not know the Duchess was there and thought the bodyguard was asking him not to photograph the players with their shirts off. But that did not make a difference to the Duchess as she only wanted to watch her pal Serena play.
There have been rumours of a royal rift between Kate and Meghan.
A spokesman for the Met Office said the duchesses ran the risk of some wet weather.
It's the first time the royal women have stepped out together this week - on Wednesday the pair joined forces to cheer on their husbands - Prince William and Prince Harry - as they took part in the King Power Royal Charity Polo Day at Billingbear Polo Club in Wokingham, UK.
It was the first official engagement Meghan and Kate had carried out together, without their partners.
During the tournament the tennis superstar spoke about how she has made sure she spent time with her daughter Olympia despite her busy playing schedule.
"We like to do a lot of sports with Olympia".
Poldark star Aidan Turner, actress Lily James and Doctor Who's Jodie Whittaker were also spotted among the Wimbledon crowds.