The Extreme Heat Policy was enacted at Melbourne Park just before 2 p.m. Thursday, suspending all matches on outer courts until the early evening and requiring the closure of the retractable roofs at Rod Laver and Hisense arenas before play could continue on the show courts.
It was the first time since 2009 play had been halted due to heat at the Australian Open.
For Maria Sharapova, playing at Rod Laver Arena, the call came too late. The rule dictates the roof can't be closed until the end of a set in progress, so she was forced to finish a grueling third set against Italian Karin Knapp under the blazing sun.
"Everyone knows there is no tiebreaker in the third set (at the Australian Open), so once you start that set, you're going to be out there until you're done," she said after closing out the 3 1/2-hour match, 6-3, 4-6, 10-8.
Other players wilted in the heat. American Varvara Lepchenko received medical treatment during her match against 11th-seeded Romanian Simona Halep, lying flat on her back during a changeover as trainers rubbed iced on her body.
"At first I didn't understand what was going on but then my legs, my arms started to get heavier.
She continued but only won one more game in a 4-6, 6-0, 6-1 defeat.
"They definitely should have just not started the matches in the first place," she said. "And the same goes for a couple of days ago when I played my (first) match."
The heat wave began Tuesday when the temperature peaked at 42C (108F). The tournament referee did not halt play then because the Extreme Heat Policy also takes into account other factors, such as humidity and wind speed.
The distinction is lost on the players, who have grumbled all week about the conditions which some have described as inhumane and dangerous. On Tuesday, Canadian Frank Dancevic blacked out and hallucinated during his match, while China's Peng Shuai vomited and suffered cramps.
No. 25-seeded Alize Cornet of France sobbed on court Thursday after her draining 6-3, 4-6, 6-4 victory over Italian Camila Giorgi, then blasted officials for not halting play sooner.
"On Tuesday, I don't know why they didn't stop matches," she said. "It was an oven. An oven. It was burning. Why today and not Tuesday?"
The heat hasn't just affected players—it's also kept fans away. Total attendance was just 49,860 on Wednesday, down from Monday's high of 63,595.
The unshaded seats on the outer courts were virtually empty Thursday, with spectators congregating under trees or in the upper reaches of stands where temporary covers provided a little relief.
New Zealander Helen Naylor escaped the sun after watching fellow Kiwi Marina Erakovic play for a bit on Court 13.
"Even the seats are really hot—God knows how (the players) are running around out there."
Relief is as least in sight. Friday will be another scorcher, with an expected high of 44C (111F), but Saturday it will only reach 23C (73F). That may feel downright chilly by comparison.
Associated Press writer Jocelyn Gecker contributed to this report.