US Open won't seek special exemption for Djokovic to play at the grand slam
The only way Djokovic could play at the US Open is if the United States of America change their immigration policy so that unvaccinated individuals are able to enter the country. That seems highly unlikely because the country has experienced a new wave of COVID cases in the past weeks which means that any policy changes won't probably take place.
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Things could change but for now, it seems very unlikely. Another way Djokovic could play is if the event applies for a special exemption for the player akin to the Australian Open however recent reports from the US suggest that the event will follow government policy and won't request an exemption for Djokovic. It basically means that as of now Djokovic won't play at the US Open with unlikely any changes happening in the future.