The world No. 1 made short work of her opponent as she lost just two points on her serve in the entire match, racing to a 6-1, 6-1 victory in 44 minutes, per the tournament's official website.
For the American, the match represented an impressive milestone in the tournament, per its official Twitter feed:
The Russian, meanwhile, was competing in just her second Grand Slam after taking part in the U.S. Open in 2015.
A loose forehand from Williams handed Kasatkina the first point of the match, but the 21-time Grand Slam winner set the tone for the contest as she roared back to claim the opening game with some strong serving and typically powerful winners.
In the second, the 18-year-old appeared to be succumbing to the pressure as she made consecutive double-faults to hand her illustrious opponent an early and easy break.
Bleacher Report columnist Merlisa Lawrence Corbett believed the tie was as good as over:
A straightforward hold to love followed as the 34-year-old raced to a 3-0 lead in the opener.
Despite producing another double-fault and allowing Williams two break points in the fourth, Kasatkina fought hard to hold her serve and claim her first game in the match.
The American's serving continued to impress as she powered her way through yet another game with consummate ease, before breaking her young opponent once more—yet again with help from a double-fault.
From there, the first set was quickly sewn up, per the Australian Open:
The second set proceeded much like the first, with WTA Insider noting Kasatkina's struggles on her serve as she was broken in the first game:
With a combination of magnificent serving, strong forehands and exquisite volleys, Williams readily dismantled her opponent. The New York Times' Ben Rothenberg ran the rule over both players:
Per BBC Sport's Andy Cryer, Williams admitted she felt good about her performance and praised her young opponent:
"I think I played well. Everything I did went right. I have so many memories of being 18, Darya has such a bright future, she is so young. Her team is doing an amazing job and she is heading in the right direction."
With Williams in such incredible form, it's difficult to see past her to claim the Grand Slam outright.
Of course, as the tournament progresses, she'll face much sterner challenges than she did here, with Margarita Gasparyan looking to provide that in the next round.