Australian Open officials urged spectators to stay in the shade and drink plenty of water Thursday with temperatures set to rocket to 39 degrees Celsius (102 Fahrenheit).
As play got underway, mechanical roofs stayed open on the Rod Laver and Hisense arenas with officials closely monitoring the air temperature and humidity.
Thousands of fans donned hats and sunglasses and slathered themselves in sunscreen, as the tannoy announcements urged them to take precautions in the extreme heat.
The Australian Open, held in the height of the southern hemisphere summer, has already tested players who have been using iced towels and extra shade during bright sun over the first three days.
Andy Murray and Roger Federer were among the players contesting 32 singles and 30 doubles matches on 16 different courts at Melbourne Park, only two of which have roofs.
Murray, the world number three, said he hoped his intense off-season training in the heat of Miami would help him deal with the conditions.
"It is on days like today -- it's predicted to reach 39C -- that I hope all the work I've done in Miami will pay off, because it's really challenging playing in that heat," he wrote in a column for The Australian.
"The key is to make sure you're rested. Yesterday I had a slightly lighter hit than usual and made sure I keep myself cool and relaxed and well hydrated. Hopefully it pays dividends today."
The Australian Open is renowned for its challenging heat and controversy hit the tournament in 2009, when officials baulked at closing the Rod Laver Arena roof despite intense conditions.
Defending champion Novak Djokovic was one player to succumb when he pulled out mid-match citing exhaustion. Days after the tournament, areas near Melbourne were hit by the Black Saturday bushfires, which killed 179.