Third seed Halep produced a nearly flawless performance on Court Philippe Chatrier against a bold opponent who turned 18 last Friday.
The Romanian will now face another teenager, American Amanda Anisimova, who thrashed Spain's Aliona Bolsova 6-3 6-0.
"I feel old," joked the 27-year-old Halep.
The former world number one said she had been coping well with the pressure of being defending champion at Roland Garros.
"I saw it different since the beginning. I felt different. I felt more relaxed, even if the pressure was on," she said.
"I feel good in this position and I always say that even if you're No. 1, you win a Grand Slam, you have to enjoy the moment.
"So I did that pretty well this year. Back to the quarter-finals is not easy, so I'm thinking just to give my best again next round. It's just what I'm thinking about."
Swiatek came out with all guns blazing but failed to master her raw power, bowing out after 45 minutes when Halep whipped a forehand winner down the line.
(Reporting by Julien Pretot; Editing by Clare Fallon)
