Serena Williams withstood a "rough and rowdy" challenge from 17-year-old American wildcard Caty McNally at the US Open on Wednesday to keep her bid for a record-equalling 24th Grand Slam title intact.

Williams, a six-time champion in New York, recovered from dropping the opening set to prevail 5-7, 6-3, 6-1 and set up a third-round showdown with either 29th seed Hsieh Su-Wei or Karolina Muchova.

"I had to stop making errors. I made so many errors in the first two sets. I was just too many. You can't win tournaments making that many errors. I knew I had to play better, and I knew I could," Williams said.

"I want to be able to win matches where I'm not playing my best, play players who are playing great, be able to come through. I need to be tested, I guess."

McNally, who hadn't been born when Williams won her first Slam at the 1999 US Open, threatened a struggling Williams from the start before the 37-year-old won nine of the last 11 games to escape under the roof at Arthur Ashe Stadium.

"I survived tonight. I'm not too pleased with the way I played at all," Williams said.

"I'd rather not be tested in every match. But that doesn't happen, so it's important for me to have those, like, really rough, rowdy matches. That helps a lot."

Eighth seed Williams is seeking to match Margaret Court's all-time record of 24 Grand Slam crowns and needed to gather her thoughts after a first set littered with 15 unforced errors.

"I definitely treasure it because it's important to have," Williams said of her ability to regroup. "I've won so many Grand Slams because of it.

"Obviously I would be so lying if I said I enjoy the tough matches more. But I think looking back years later, I enjoy the tough matches more. But in the moment, in the weeks after, it's definitely a different feeling."