Tennis star Nick Kyrgios committed a huge COVID no-no on Tuesday -- celebrating his Australian Open win by taking a swig of a fan's beer.
Check out the clip ... after Kyrgios defeated his opponent, Liam Broady, in the first round, he went over to some fans and had a beer put in his face.
The supporter holding the suds appeared to urge Nick to drink -- and after a few seconds of debating it, he caved and took a sip from the guy's cup.