The French Open champion 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 feisty 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 whacked a backhand into the net.

In the end, Swiatek's superior serving and relentless defense carried the day against Davis, who plays a similar but less potent brand of tennis as Swiatek.

The 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 in the US Open fourth round after the Wimbledon quarterfinalist upset China's Qinwen Zheng 6-4 7-6(5) to reach the Round of 16.