Serena Williams and Novak Djokovic were on the official US Open entry lists released Wednesday, but that doesn't mean they'll necessarily play in the tournament -- and in Djokovic's case, he can't, as of now, because he isn't vaccinated against COVID-19.

The singles entry lists are a formality, and they include all players who are eligible based purely on this week's WTA and ATP rankings. Williams is on there by virtue of a special ranking granted to her because she missed so much time because of injury; her first match in a year came last month at Wimbledon, where she lost in the first round to Harmony Tan.

Asked after that defeat whether she would play again, the 40-year-old Williams said she didn't know. When she was asked specifically about returning to the US Open, where she has won six of her 23 Grand Slam singles trophies, Williams replied: "There's definitely lots of motivation to get better and to play at home."

She missed the hard-court tournament in New York last year after injuring a leg at Wimbledon. But Williams made it to at least the semifinals in each of her 11 most recent appearances at the US Open.

Djokovic, a 35-year-old from Serbia, missed the Australian Open in January after being deported from that country over his vaccine status and has said since that he will not get COVID-19 shots just to be able to participate in tournaments.

The United States does not allow unvaccinated noncitizens to enter the country, which is why Djokovic needed to sit out big events in Indian Wells, California, and Miami earlier this season, and can't compete at the Cincinnati Masters next month or the US Open, which begins Aug. 29.

In its news release announcing the entry lists, the US Tennis Association said: "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."

Djokovic was able to play in the French Open, where he lost in the quarterfinals to eventual champion Rafael Nadal, and at Wimbledon, which he won for his 21st Grand Slam title.

Only Nadal, with 22 major championships, has more among men in the history of tennis.

After his triumph at Wimbledon, where he beat Nick Kyrgios in the final, Djokovic said that he "would love" to enter the US Open, but he added: "I'm not planning to get vaccinated."

Djokovic has won three championships at Flushing Meadows and has been the runner-up a half-dozen times, including a year ago. His loss in the 2021 final to Daniil Medvedev prevented Djokovic from becoming the first man since Rod Laver in 1969 to win a calendar-year Grand Slam.