Djokovic will not be able to participate in the year's final Grand Slam due to current rules under which travellers seeking to enter the United States have to be fully vaccinated and provide proof before boarding flights.
"The US Open does not have a vaccination mandate in place for players, but it will respect the US government's position regarding travel into the country for unvaccinated non-US citizens," the United States Tennis Association (USTA) said in a news release.
Almost 12,000 people have signed an online petition calling for the USTA to work with the country's government to allow Djokovic to compete in the US Open despite his refusal to take the COVID-19 vaccine.
Djokovic won the Australian Open, French Open and Wimbledon titles in 2021 but was unable to defend his Melbourne Park crown this year after being deported from Australia over his vaccination status in January.
He later said he was prepared to miss Grand Slam tournaments rather than take the COVID-19 vaccine.
Williams, who returned to competition last month after a year-long absence, was added to the entry list for the August 29-September 11 Grand Slam in New York having been named in the draw for tune-up events in Toronto and Cincinnati.
The 23-times Grand Slam champion fell in the first round at Wimbledon and last appeared in the US Open in 2020 when the six-time winner reached the semi-finals.
Eligible players are automatically entered into the main draw but that does little to help Djokovic, who retained his Wimbledon crown last month and is one back of Rafael Nadal's record of 22 Grand Slam titles.
