Six-time champion Novak Djokovic came back from a set down to beat Gael Monfils as the temperature hit 39C at the Australian Open.
Monfils took the first set but began to suffer in the second, complaining of feeling dizzy and leaving the court with a doctor.
The Frenchman returned to Rod Laver Arena but could not stop Djokovic coming through 4-6 6-3 6-1 6-3.
The temperature is forecast to exceed 40C in Melbourne on Friday.
“It was obvious that we both suffered on the court today,” said Djokovic. “Really tough conditions, brutal, especially for the first hour and a half.”
With the temperature peaking at 17:00 local time, the pair entered the arena mid-afternoon and played through the worst of the heat.
Djokovic could not have asked for a more punishing examination of his fitness after six months out with an elbow injury.
Asked afterwards about his elbow, Djokovic said: “It’s still not 100%, but building.”
Seeded 14th after sliding down the rankings during his lay-off and with a new service action to take pressure off his elbow, he found himself up against an in-form Monfils hopeful of finally beating the Serb at the 15th attempt.
The former world number one began poorly, making four double faults in the opening two games as he fell a double-break down at 3-0, but the set was far from over.
With both men spraying errors – they shared 36 between them across the first set – it was a question of who would fall over the line and clinch the first set.
Monfils managed to hold on after seeing his 3-0 lead disappear, edging through another tense service game and wasting his first set point with a woeful drop shot.
It was midway through the second set that the Frenchman started to struggle with the conditions, and Djokovic took advantage.
He broke the ailing Frenchman’s serve three times in the third set and saw off five break points on his way to clinching victory in the fourth.
“I knew it was going to be a big challenge for both of us,” Djokovic added.
“Gael is one of the best athletes in our sport. I was just hanging in there, trying to use every single opportunity that is presented.
“He wasn’t at his best at the end of the second and entire third set. The fourth could have gone either way.”