Serena Williams and Novak Djokovic were on the official U.S. 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 due to injury; her first match in a year came last month at Wimbledon, where she lost in the first round to Harmony Tan.