The four-times champion, who has lost only twice at Roland Garros since 2020, had far too much quality for her opponent but made 38 unforced errors, including a series of double faults.
Swiatek, the world No.3, explained her lack of precision by the nature of the opposition.
"She was pretty good in defence, and I had many opportunities to attack. Sometimes even too many. So when you attack, you're going to make more mistakes, because it's easier to push the ball back than to actually add a lot of energy to it and try to open the court," the 24-year-old said.
"So, yes, sometimes I made some mistakes while playing these high balls, because the ball is jumping quite high. It's not easy to judge if it's the one that you should go or stay back and wait for the next one.
"The rhythm wasn't quite obvious. But I didn't mind. I wanted to be flexible and adjust and play my game. I had some moments where the mistakes happened, but at the end I feel like I was the one who had control over the game, and it was up to me if I'm going to finish these points or make a mistake."
Swiatek will now face either former champion Jelena Ostapenko or fellow Pole Magda Linette for a place in the last 16.
After an early exchange of breaks, Swiatek surged into a 5-1 lead on Court Philippe Chatrier before gifting world No.35 Bejlek another break with a double fault in the seventh game.
The Pole repeatedly confused pace with haste but still claimed the opening set comfortably despite spraying 19 unforced errors.
Five of the opening eight games in the second set went against serve in a scrappy contest before Swiatek finally sealed victory when Bejlek dumped a forehand into the net.
Elsewhere, Elina Svitolina extended a run of solid form with a 6-0 6-4 victory over Kaitlin Quevedo. Svitolina is on an eight-match winning streak after lifting the Italian Open trophy.
Switzerland's Belinda Bencic, the 11th seed, beat American opponent Caty McNally 6-4 6-0.
