(CNN)Serena Williams is included on the entry list for this year's US Open, along with 21-time grand slam champion Novak Djokovic, who is currently unable to enter the country due to rules that require travelers to be fully vaccinated against Covid-19.
Novak Djokovic and Serena Williams on US Open entry lists -- but that doesn't mean they'll play
However, the inclusion doesn't mean that either will definitely play.
Williams returned to grand slam action this month at Wimbledon after a year-long break -- but was knocked out in the first round after a three-hour, 11-minute tussle with France's Harmony Tan.
But Williams, who turned 40 last September, is refusing to give up on her pursuit of a record-equaling 24th grand slam title, five years after she claimed her last at the Australian Open. She is currently entered in tournaments that will take place later this summer in Toronto and Cincinnati.
Meanwhile, more than 23,000 people have signed an online petition on change.org calling for the United States Tennis Association (USTA) and the US government to allow Djokovic to compete in the grand slam.
Djokovic's refusal to take the Covid-19 vaccine has hindered his participation in tournaments around the world, with the Serb unable to defend his Australian Open title in January after being deported following a period in detention.
He later told the BBC that he would be willing to miss tournaments to maintain his unvaccinated status and he has forgone playing in several tournaments throughout the year, including those based in the US, such as the Miami Open and Indian Wells.
He has played in two grand slams this year -- the French Open, where he lost to eventual champion Rafael Nadal in the quarterfinals, and Wimbledon, where he won his fourth edition in a row with victory over Australia's Nick Kyrgios in the final.
"Per the Grand Slam Rule Book, all eligible players are automatically entered into the men's and women's singles main draw fields based on ranking 42 days prior to the first Monday of the event," the USTA said in a statement Wednesday.
"The US Open does not have a vaccination mandate in place for players, but it will respect the U.S. government's position regarding travel into the country for unvaccinated non-U.S. citizens," it added.
Currently unable to enter the United States -- and with his participation in the Australian Open also in doubt due to laws that could ban him from entering the country for a further three years -- Djokovic's next appearance at a grand slam could be the 2023 French Open.