It was not a major surprise though as Stephens has reached the fourth round of the claycourt grand slam in the past three years. She will face Briton Heather Watson next.
Seven-times grand slam champion Williams, whose best result in Paris is a runner-up spot in 2002 but had not gone past the second round in the last four years, made the better start, opening a 2-0 lead but Stephens proved more accurate.
The 22-year-old broke back and sent the opening set into a tiebreak, which she easily won as her opponent, whom she had never met, netted a forehand.
The 34-year-old Williams never recovered and looked short of energy in the second set as Stephens raced to a 3-0 lead and never looked back, ending the baseline contest with an ace.
"I feel like I had some good practices coming into Roland Garros but sometimes things don't always work out the way you would like," said Williams.
Stephens's biggest fright of the match came in the second game of the second set, when a pigeon came down flying past her as she was about to receive a serve.
"First of all, I didn't know what it was," an amused Stephens, who reached the semi-finals at the Strasbourg Internationals last week, told a news conference.
"I thought it was way closer to me than it actually was. So that was why I screamed so loud. And I don't know, in the moment, things just happen and it was just scary and I was like, ahhh and a scream came out.
"And then she probably wasn't happy because she probably would have hit an ace and it was just a weird moment."
(Reporting by Julien Pretot; Editing by Martyn Herman and Justin Palmer)
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