As of right now, Novak Djokovic won't be able to play at the US Open due to the country's rules regarding COVID.
As an unvaccinated foreign national, Djokovic is not permitted from entering the US per the country's rules. He could enter the countries with certain exemptions however the US Open is not willing to apply for it and Djokovic from his side won't be taking any vaccine in the future. That makes his participation practically impossible unless a certain person in the US decides otherwise.
The person in question is president Biden who could exercise his powers to allow Djokovic to enter the country under the pretext of 'national interest'. This was pointed out by Serbs living in the US who sent a letter to the president explaining the situation. Nebojsa Jovanovic, a Serb in the US consulted with the Serbian-American Voter Alliance before sending the letter.
According to Jovanovic, Article 3(b) of the proclamation allows for exemptions of certain foreign nationals from existing Covid protocols, if that is considered to be in the US national interest. In addition to President Biden, this exemption can also be granted by Secretaries Blinken, Buttigieg, or Mayorkas, who have also been addressed.
Part of the letter to prsident Biden reads:
"The US Open is the biggest tennis tournament in the world, and it will not be what it is without Novak Djokovic. Considering the economic situation and the crisis at all levels, America must not allow the US Open not to be held in its full glory and capacity, which also means the participation of Novak Djokovic, who has been crowned three times at the US Open. Novak does not pose any security threat, is one of the healthiest people in the world due to his disciplined lifestyle and serves as a role model and inspiration to millions around the world. It is in America's interest that the biggest tournament in the world hosts the best player in the world, especially since he just won his seventh Wimbledon title in London."
It's unlikely to work considering the strong stance the Biden administration has taken against the virus with a heavy emphasis on vaccination however fans of Djokovic are still trying to get him there.