Swiatek survives surprise test from Niemeier to reach US Open quarters
Swiatek, who collected her second Roland Garros title amid a 37-match winning streak earlier this year, barely looked like the world's best player as she sprayed the court with errors to surrender the opening set.
She handed Niemeier a break in the third game of the second set with a clumsy mistake at the net. Sitting on her bench with her towel over her head during the change-over, she loudly smacked her thigh in anger before returning to the court with new resolve, winning the next three games.
The pair traded breaks twice to close out the second set but Niemeier scarcely put up a fight in the third, producing 14 unforced errors as her game quickly unravelled.
Swiatek, who won in Doha, Indian Wells, Miami and Rome this year, sounded rattled after the match as her usual legion of Polish supporters rallied around her from the stands.
"I'm just proud that I didn't lose hope," she said in an on-court interview. "I had hard time like pushing her back."
It is the first time she has reached the quarter-finals at Flushing Meadows. She next faces in-form American Jessica Pegula (28).
"I'm really happy to play in front of you guys and I hope my matches are going to be good to watch so you're not going to waste your time here," she said.