The Pole, who was on a 25-match winning streak at Roland Garros, was a clear favourite to win the gold medal after a third consecutive French Open title this year.
"I had a hole in my backhand," she told Eurosport Poland. "It happens rarely because it is usually my most solid stroke."
Swiatek held a 6-0 career record against Zheng, but the clay-court specialist produced one of her worst displays of the season, making 36 unforced errors against her opponent's 15.
The five-time Grand Slam champion blamed stress and a lack of time between matches for her performance.
"I was not technically well positioned because of the stress and the fact that I played my games day by day. We didn’t have time to adjust that and work on that," Swiatek said.
"I know that’s not the justification but I tried to correct that during the match. Today it didn’t work at all. So she used that to win the game," she said
Swiatek refused to take questions from international reporters after the match. "Sorry, next time," she said.
It has been a tough week for Swiatek who was hit by a ball in her quarter-final win over Danielle Collins on Wednesday.
Collins accused the Pole of being "insincere" after the American abandoned the match due to stomach pain.
Collins blamed her injury on the lack of fresh water on the courts, which she said provoked heat stress and convulsions, amid extremely hot weather.
The Pole still has a chance to win a medal and will play the loser of the other semi-final between Croatian Donna Vekic and Slovak Anna Karolina Schmiedlova.
