Williams, who mixed 24 winners with 33 unforced errors over the contest, has not won a major since her seventh Australian Open triumph in 2017 and next faces in-form Japanese third seed Naomi Osaka with a place in the final on the line.
For Halep, who reached the semi-finals last year and was losing finalist in 2018, it was back to the drawing board as she looks to add to her two Grand Slam titles at the French Open and Wimbledon.