Swiatek won just over half of her first serve points and faced eight break points in the match at Arthur Ashe Stadium. She also had 41 unforced errors.
"When I (was) losing 4-0 in the first set, I tried to do the same kind of work as usual," said the 2022 champion, who had won six of her previous nine matches against Pegula.
"I didn't really understand why my serve wasn't working. It was hard for me to find, like, a proper solution for that."
The five-time Grand Slam winner said Pegula was a worthy winner.
"She has a tricky ball because it's pretty low and pretty flat ... she was for sure more solid than me and making much less mistakes. So because of that, she was kind of putting pressure with that," added the Pole.
"But usually I'm able to push it back or put pressure on myself, but today I just made too many mistakes."
Swiatek, who won her third straight French Open in June before taking bronze in the Paris Olympics last month, said she did not want to take a break after the final major of the year.
"It could be a little bit easier, but I'm keeping up with the schedule. I'm only talking about mandatory tournaments and there are rules about mandatory tournaments," she added.
"But I'm ready for playing till November, I guess, unless I'm going to get injured."
