Iga Swiatek covered her head with a towel in the changeover after falling behind by a set and break. She made mistakes and then looked in the direction of her guest box.
After getting her strokes straightened out, Swiatek was able to play more like someone who is ranked number one.
Swiatek said that he did not lose hope.
Another first-timer from Poland will face the 21-year-old in the quarterfinals. The highest-ranked American woman, who advanced earlier Monday with a 6-3, 6-2 victory over two-time Wimbledon winner Petra Kvitova in a match interrupted by rain for 45 minutes, is Jessica Pegula.
It was difficult at the start of the match, when there wasn't a sense of what was happening. You are not sure what to do.
There was no rain predicted for the U.S. Open, according to the U.S. Tennis Association. There was a pop-up sunshower.
The other women's fourth-round matches Monday were two-time Australian Open champion Viktoria Azarenka against two-time major finalist.
The last three games of the opening set were won by the son of the owners of the Buffalo Bills and NHL's Buffalo Sabres.
This will be the third time this season that Pegula has faced the woman who led the rankings. At the Australian Open, she lost to Ash Barty, who went on to win the title, and at the French Open, she lost to Swiatek, who went on to win the title.
"I have to be present and play within myself," said the man.
Swiatek arrived in New York having not won a match since the end of her winning streak. She came out flat against her opponent, who reached the quarterfinals at Wimbledon in July, after listening to her usual pre- match music.
The first set was a good one. Better than before. Swiatek produced only four winners and 13 unforced errors and never earned a break point.
Swiatek had to win the second set to pull even.
She draped herself with a towel after falling behind. She talked to herself under there.
Swiatek immediately took control with a three-game run that gave them a 4-2 lead. She double-faulted to end that game after getting a chance to serve.
It looked like nothing would come easily on this hot day, with the temperature above 80 degrees and the humidity above 50%.
Swiatek broke right back, aided by a wild forehand from the other side of the net.
Swiatek's unforced error count was one for her and 14 for the other. Swiatek shook her fist after jumping and throwing an uppercut.