... and Serena Williams loved every second of it!!!

Don't worry -- this wasn't a super-spreader event ... because of strict quarantine rules and other regulations in place to fight COVID, Australia is actually a coronavirus-free area.

In fact, officials announced Friday the entire country hasn't had a positive case in nearly two weeks!!!

So, an estimated crowd of around 4,000 jammed into an arena in Adelaide on Friday to watch tennis stars like Serena Williams, Naomi Osaka and Novak Djokovic prep for the Aussie Open ... and it was all awesome.

Check out the pics ... fans were shoulder to shoulder, with no masks and no social distancing -- and no worries at all of getting sick.

As for the players, they were all forced to undergo a 14-day quarantine upon arriving in Australia to ensure they didn't spread the virus ... and most had expressed publicly how difficult that was.

Williams, though, spoke after her match with Osaka and was grateful the rules and regulations allowed her and others to play maskless and without fear of catching COVID.

"We haven't played in front of a crowd in over a year," Williams said. "So it's been a really long time. So this is really cool."

Serena thanked the Australian crowd for "having us and trusting us with your laws" -- adding, "We were so happy just to be here and now it's worth it."