Iga Swiatek said she struggled for "rhythm" in her US Open third round victory over American Lauren Davis on Saturday as the world No 1 booked her place in the last 16.
The French Open champion needed to dig deep in the second set before claiming a 6-3, 6-4 to reach the fourth round for the second straight year.
Swiatek came out sharp and fired an unreturnable serve out wide to capture the first set before falling behind 4-1 in the second as Davis raised her level.
Despite the deficit the Pole stubbornly refused to drop her first set of the tournament, winning a tense 16-shot rally to set up break points at 4-4, which she converted when Davis hit a backhand into the net.
In the end, Swiatek's superior serving and relentless defence carried the day against Davis, who plays a similar but less potent brand of tennis as the top seed.
The 21-year-old top seed admitted she was not at her best during the two-hour night match on Louis Armstrong Stadium despite reeling off the match's final five games.
"I couldn't find my rhythm today," Swiatek said in an on-court interview. "She played totally differently than any other player. She played very smart. I'm pretty happy at the end that I could put balls in.
"I'm trying to enjoy every match even when it's not perfect. Just trying to go for it. Trying to find solutions in every situation."
Swiatek went on a blistering 37-match winning streak earlier this year to claim titles in Roland Garros, Doha, Indian Wells, Miami and Rome but cooled off this summer, losing four matches during the hard-court season.
So far in New York she has put to rest any questions about her form and will be the heavy favourite when she takes on Germany's Jule Niemeier on Monday after the Wimbledon quarterfinalist upset China's Qinwen Zheng 6-4, 7-6.
Also through to the women's fourth round are Belarusian sixth seed Aryna Sabalenka, who thrashed France's Clara Burel 6-0, 6-2, and American Daniel Collins following her 6-4, 7-6 win over another Frenchwoman, Alize Cornet.
The pair meet each other next for a place in the quarterfinals.