Some of the greatest tennis players EVER -- from Serena Williams to Roger Federer -- hit the court Wednesday ... and it was all to help raise nearly $5 MILLION for Australia fire relief!

With the Australian Open set to kick off next week ... legends like Rafael Nadal and Novak Djokovic joined Serena and Federer at Rod Laver Arena in Melbourne to get some pickup games in for charity.

The event was great ... the superstars hammed it up for the crowd, got in a little tennis action and, most importantly, raised roughly $4.8 mil to help victims of the raging wildfires.

As we previously reported, more than 2,500 homes have been destroyed since the blazes began ... with the fire killing at least 28 people.

The tennis world has been super active when it comes to helping relief efforts ... with stars like Nick Kyrgios famously pledging $200 per ace to the cause this season.

As for the Australian Open event, it's still going down as scheduled for now ... although players have already expressed concern over the poor air quality.

"I think there has to be some kind of line in the sand, you know, some kind of rule where you measure the air," Genie Bouchard told Gizmodo this week ... "And if it’s over a certain number, or however they measure it, then you just don’t play."

The Open matches are set to begin in Australia on Monday.