The International Tennis Federation (ITF) has taken the decision to push the start of matches at the Tokyo Olympics to 3pm local time after frequent complaints from players regarding the heat and humidity.
Men's world No 2 Daniil Medvedev has been the most outspoken critic of the conditions and on Wednesday, the Russian told the umpire he could finish his third round match but wanted to know who would take responsibility if he died.
World No 1 Novak Djokovic has also questioned the scheduling, while Medvedev's compatriot Anastasia Pavlyuchenkova complained about the conditions following the first round when she needed to take a medical timeout.
The ITF had previously resisted calls to move the schedule to a later time of the day, but on Wednesday acted "in the interests of player health and welfare".
"The ITF has announced a change of schedule due to the increasing heat and humidity currently being experienced in Tokyo," the governing body said.
Medvedev opened proceedings at 11am on Ariake Tennis Park's main show court against Italian Fabio Fognini.
The 25-year-old took a medical timeout on court and called the trainer on two other occasions, and chair umpire Carlos Ramos asked Medvedev if he was alright during the second set.
"I'm fine. I can finish the match but I can die. If I die will the ITF take responsibility?" Medvedev replied.
The ITF's "extreme weather policy" allows a 10-minute break between the second and third sets if agreed by both players and Medvedev used that time to get a freezing cold shower.
"Even from the first set I didn't feel good enough with my breathing. That's why I called the physio, I felt like my diaphragm was blocked," he said.
"And then in the second set, I just had darkness in my eyes, like between every point, I didn't know what to do to feel better."
While Medvedev said he was "lucky" he could continue his match to advance, Spain's Paula Badosa gave up after suffering heat stroke during her match against Czech Marketa Vondrousova.
She needed a lengthy medical timeout but was unable to recover and left the court in a wheelchair.
"It's a shame to end my participation in this way. It's been a tough ask since day one, we tried to adapt as best we could but today my body hasn't held up as it needed to," said Badosa.
"I suffered heatstroke ... I didn't feel like I could carry on with the game."
Djokovic won his singles match in better conditions in late afternoon before returning to register a mixed doubles victory.
"Tennis needs to be considered as one of the endurance sports," the 20-time Grand Slam winner said. "I am playing tennis professionally for 20 years and I've never faced these kind of conditions in my entire life on a daily basis."