AO officials made the announcement Wednesday ... explaining an employee at one of the Australian Open quarantine hotels came down with the virus this week.

And, because of the case, they've now canceled Thursday's Australian Open tune-up games ... and have said players and AO workers who were at the hotel must now go back into quarantine.

"Those associated with the AO who quarantined at the hotel now need to be tested and isolate until they receive a negative test result," officials said.

"We will work with everyone involved to facilitate testing as quickly as possible. There will be no matches at Melbourne Park on Thursday."

Of course, Australia's COVID rules required everyone who flew in from out of the country to isolate for 14 days AND test negative before they could go out in public.

The rule had seemed to work beautifully ... Australian officials had just announced they hadn't had a positive case in the entire country in nearly 2 full weeks.

In fact, things were going so swimmingly, maskless fans packed an arena last week to watch some Serena Williams, Naomi Osaka and Novak Djokovic warm-up games.

Now, however, the AO's Feb. 8 start time could be in jeopardy due to the new case ... although officials said there's no change on that front as of this moment.