US Open organizers say they are confident in safety plans a month ahead of the Grand Slam tennis event in New York even as US COVID-19 outbreaks increase.

The US Tennis Association (USTA) said Friday it "continues its plans to stage the US Open" as well as an ATP and WTA tuneup event the week before the Flushing Meadows fortnight.

The US Open is scheduled to begin August 31 in a bubble quarantine setting without spectators at the National Tennis Center.

"We remain confident that our top priority, the health and safety of all involved in both tournaments, remains on track," the USTA said in a statement.

Just what those health and safety details are, however, remains unknown.

The USTA says it will release details of its health and safety plans "at a later date closer to the tournaments" but notes it has worked with both tours in all aspects of health and safety.

Novak Djokovic and Rafael Nadal have entered the Western and Southern Open, the tuneup event normally staged in Cincinnati, but said nothing certain about the US Open.

Serena Williams and Naomi Osaka have committed to playing in the US Open but world number one Ashleigh Barty of Australia says she will skip the tournament.

The New York area was the first major US hotspot for COVID-19, conditions so tough that a temporary hospital was established on the grounds of the US Tennis Center.

But cases have been kept down in recent months, although there has been concern people visiting from other outbreak areas could raise New York COVID-19 levels.

"Working with our Medical Advisory Group and security team and the State of New York, we have developed a strong health and safety plan to mitigate the risk of infection within the contained environment comprised of the tournament site and player hotels," the USTA said.

"New York State continues to be one of the safest places in the country as it relates to the COVID-19 virus."