It was an overpowering performance from Williams, who finished the quarterfinal match with significant advantages in winners (24-9), aces (4-0) and percentage of points won on the first serve (77-52).
The Grand Slam tournament's No. 10 seed, who's seeking her 24th major singles title, moves on to face No. 3 seed Naomi Osaka in the penultimate round.
Halep didn't play poorly, converting three of her four break-point opportunities and showcasing her strong counterattacking skills to win 38 percent of her return points against a typically dominant Williams serve, but it wasn't quite enough.
Williams moved within two wins of tying Margaret Court for the most Grand Slam singles titles in history at 24. Her last major triumph came in the 2017 Australian Open. She's reached the finals four times since then but fell short on each occasion.
The 39-year-old may be peaking at the perfect time based on her strong outing against Halep, who defeated her in the 2019 Wimbledon final.
"I definitely think this is the best match I've played this tournament for sure," Williams told reporters. "Obviously I had to, going up against the No. 2 in the world. I knew I had to do better and that's what I did. I've been watching her and I'm sure she's been watching me."
She'll need to continue building off that performance as she prepares to face Osaka, one of the few players on the WTA Tour with a winning record against Williams (2-1).
Those victories came in 2018, though. The sport's longtime gold standard won the most recent official meeting in the Toronto quarterfinals in 2019.
Osaka enters the latest matchup coming off a straight-sets win of her own over unseeded Su-Wei Hsieh (6-2, 6-2).
"I feel like this is such a good opportunity for me just to keep doing my best," Williams said. "It's the first Grand Slam of the year for me. I had an incredibly long, arduous offseason. And so, I'm honestly happy with every win I get."
Williams and Osaka will face off Thursday in the semifinals. A start time will be announced following Wednesday's play, which features the other two women's quarterfinals.