Djokovic missing U.S. Open over COVID vaccine status 

Four-time U.S. Open winner John McEnroe says it's "a joke" if Novak Djokovic will not be able to play in this year's U.S. tournament

starting later this month because of the COVID-19 vaccine status. 

Djokovic has refused to get the vaccine and is likely to miss the tournament 

that was last year's Grand Slam due to current U.S. rules that require participants

to present evidence of their full vaccination before they can take flights into and out of to the United States.

The Serbian previously said he was willing to skip Grand Slam tournaments

that require participants to be vaccination-free against COVID-19.