Meghan, Duchess of Sussex, and Catherine, Duchess of Cambridge, watched the clash from the Royal Box, alongside Catherine's sister, Pippa Middleton.
Williams, who was defeated by Halep, is a close friend of Meghan and attended the former actress' wedding to Prince Harry last year. She also co-hosted a lavish baby shower for the Duchess of Sussex in New York City earlier this year before the birth of Archie Harrison Mountbatten-Windsor.
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.
During her first appearance, the Duchess of Sussex, dressed in a blazer and blue jeans, appeared to be wearing a necklace adorned with an "A," -- the first initial of her son's name, while seated with two friends among rows of empty seats.
However, her Wimbledon appearance wasn't without incident, PA reported, after royal protection officers asked spectators to refrain from taking photos of the Duchess of Sussex because she was attending in a "private" capacity.
The duchesses attended last year's final together, where they watched Angelique Kerber win her first-ever Wimbledon title after a straight-forward victory against Williams.
Meghan and Catherine were spotted together earlier this week when the Duchess of Sussex took her two-month-old son, Archie, to his first polo match -- in one of his first outings since birth.
The pair cheered on Princes Harry and William as the siblings played against each other at Billingbear Polo Club in Surrey, England.
Catherine also attended with her children, George, Charlotte and Louis.