That's been bad news for the tennis players at the Australian Open, where Thursday was the hottest day so far.
There were the unlucky morning starters, like Maria Sharapova who played in 43 degree C (109 F) heat and cooled off at changeovers with ice vests on their bodies and ice collars around their necks. Some called trainers to complain of head spins, blurred vision and cramping from dehydration.
By mid-afternoon, the Australian Open invoked its "Extreme Heat Policy" and suspended play for four hours on outdoor courts and closed the retractable roofs at Rod Laver and Hisense Arena. That allowed other stars like No. 1 Rafael Nadal, No. 6 Roger Federer and No. 2 Victoria Azarenka to play indoors.
"I never actually experienced the heat today," said Federer, who advanced easily to the third round.
Later in the day after temperatures dipped, Sloane Stephens looked up to see "crazy, ridiculous" lightening zig-zagging through the sky, which caused another delay.
Here are five things to know about how the heat effects the Australian Open:
GUIDELINES FOR HALTING PLAY: Matches can be stopped and the tournament's two retractable roofs closed at the discretion of tournament director Wayne McKewen.
THE CONTROVERSY: The heat and the tournament's delayed decision to halt play sparked debate about whether it was dangerous to have allowed matches since Tuesday when the heat wave started.
A few thoughts from those who endured the heat, and those who did not.
No. 25-seeded Alize Cornet of France sobbed on court, 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?"
Anastasia Pavlyuchenkova said it was "ridiculous" to have played outdoors Thursday morning.
American player Varvara Lepchenko said her body broke down during her three-set loss. She felt "dizzier and dizzier," she couldn't see the ball. After the match she took an ice bath, drank a lot of water. "And I just laid down in the locker room for the past hour because I just couldn't physically get up."
The No. 2-seeded Azarenka played under the roof in the evening. "It was a little bit humid," she said. "But I can't complain. I'm sure a lot of players were suffering a little bit more."
HOW MANY MATCHES WERE EFFECTED THURSDAY: Twelve matches scheduled for Thursday were postponed and rescheduled, including the doubles duo of Serena and Venus Williams, who have won 13 Grand Slam doubles titles together.
ATTENDANCE: The heat has thinned out crowds at Melbourne Park. Attendance peaked for the two sessions on Monday, Day 1, at 63,595 when it was sunny with a high of 31 Celsius (88 Fahrenheit).
The heat wave arrived Tuesday with a hot breeze and high of 42 Celsius (108 Fahrenheit). Attendance dipped to 53,627.
A smaller crowd totaling 49,860 came Wednesday.
Thursday was the hottest day so far, and the numbers picked up to 53,226 due to a slightly larger night attendance.
WHAT DOES TOMORROW HOLD?: Bad news for playing tennis outside on Friday. The forecast is for a high of 44 C (111 F).
But on a brighter note, a cold front moves in Saturday with the forecasters predicting a maximum of 23 C (73 F).