Williams, 39, will play third seed Naomi Osaka on Thursday after the Japanese 23-year-old dismissed Taiwan's Hsieh Su-wei 6-2, 6-2.
Osaka hit 24 winners as she swept past the 35-year-old who was playing in her first Grand Slam quarter-final.
"I told myself just to be really intense from the beginning," said Osaka. "I felt like I knew what to expect and that I couldn't afford to be lazy with my footwork or anything."
After her victory over Halep, Williams said she had also been on her game.
"Definitely think this is the best match I have played this tournament, for sure," said the 10th seed who is seeking a record-equalling 24th Grand Slam singles title.
"Obviously I had to play well. I was going up against the number two in the world, so I knew I had to do better."
Reporter ✍️: "When was the last time you felt those rallies belonged to you"@serenawilliams 👑: "The summer of 1926 was the last time I felt that...." 😂🤣#AusOpen | #AO2021 pic.twitter.com/MZnRkfct1G
— #AusOpen (@AustralianOpen) February 16, 2021
