The top seeded American needed just 70 minutes to dispose of the world's top doubles player, Roberta Vinci of Italy, 6-1, 6-3 and in four matches at Roland Garros this year, she has dropped just 10 games.
The win puts her into a quarter-final matchup against unseeded Svetlana Kuznetsova of Russia who won the title in Paris in 2009 after defeating Williams in a quarter-final thriller.
Kuznetsova, who outlasted eighth seed Angelique Kerber of Germany 6-4, 4-6, 6-3, said she would relish going up again against Williams, even given her current form.
"But I believe that I have the game and my good days, as well. Let's cross fingers I will have a good day on that one.
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"She's just a fighter. She's unbelievable, you know. It's an unbelievable competition to compete against her and I just will try to do my best."
Williams' win also takes her career-best winning streak to 28, the third longest since 2000 behind sister Venus (35) and Justine Henin (32).
As in the previous three rounds Williams asserted her authority from the start, jumping out into a 4-0 lead. The only trouble she occasionally had came from the windy conditions that disrupted her service action.