Australian Open Tennis Halted By Extreme Heat
Matches on the show courts are continuing after the retractable roofs were closed.
Australian Open bosses invoked part of the special conditions before play began, allowing women's matches to have an extended break after the second set and players to wear ice vests during changeovers.
Temperatures are forecast to reach 44C (111F) at Melbourne Park.
Varvara Lepchenko was the player to struggle most, the American taking a medical time-out during the second set of her second-round match against Simona Halep.
Lepchenko lay down across two chairs while a doctor examined her and had ice rubbed over her body before continuing. Having won the first set she managed only one more game in a 4-6 6-0 6-1 defeat.
Lepchenko said: "I think they definitely should have just not started the matches in the first place. In the second set I couldn't focus on my returns, I couldn't see the ball."
No 25 seed Alize Cornet, of France, sobbed on court after her draining 6-3, 4-6, 6-4 victory over Italian Camila Giorgi, which lasted two-and-a-half hours.
"I think I spent enough time on this court for today," she said in her post-match, on-court interview.
"I went really further than my limits. It was really hot, that's why I'm so emotional.
"Doing something physical in this heat it's just unbelievable. Even you guys (in the crowd) sitting in the sun, it must be terrible."