Serena Williams admitted she would rather not be tested in every match at the US Open after a "really rough, rowdy" win over Caty McNally on Wednesday.

The 23-time grand slam champion enjoyed a serene start at Flushing Meadows this year, easing past 2006 champion Maria Sharapova, but found life tougher against teenager McNally.

Errors dogged Williams as she dropped the first set and had to respond strongly to advance to the third round with a 5-7 6-3 6-1 victory.

The 37-year-old acknowledged the need to come through difficult matches such as this one but hoped she would not need to dig quite so deep every time during her bid for glory in New York.

"I think it's great because 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," she told a news conference.

"I need to be tested, I guess. That sounds scripted - actually, I'd rather not be tested in every match.

"But that doesn't happen, so it's important for me to have those really rough, rowdy matches. That helps a lot."

Williams suggested the victory over McNally was the type of win she would later be able to reflect on with greater positivity.

"Obviously I would be lying if I said I enjoy the tough matches more," she said. "But I think looking back years later, I enjoy the tough matches more.

"In the moment, in the weeks after, it's definitely a different feeling."

McNally won the girls' doubles title alongside Coco Gauff at the US Open last year and was delighted with the way she dealt with the step up in quality and experience against Williams.

"I think it just shows that I'm capable of playing with the best," the 17-year-old said. "I got a set off Serena Williams. I had her close in the second set, too, had some chances.

"For me, it just gives me a lot of confidence, shows me that I can compete out on the biggest stages. I'll just keep my head down, I'm ready to go back to work."