Organisers acceded to player requests to change the tennis schedule at the Tokyo Olympics after Spain’s Paula Badosa suffered heatstroke and left the court in a wheelchair.
Badosa needed treatment after losing the opening set of her singles quarter-final 6-3 to Naomi Osaka’s conqueror Marketa Vondrousova.
She later recovered but pulled out of her mixed doubles opener alongside Pablo Carreno Busta, with her coach Javier Marti saying: "The conditions today were so tough to play in.
"In her last two points, she was feeling very bad, she was serving very slow. After she asked to get treatment, she had no energy left and couldn’t continue as she was feeling dizzy."
Men’s second seed Daniil Medvedev also struggled and was heard to say during his third-round match against Fabio Fognini: "If I die, who will take responsibility?"
A number of players have complained about being forced to play in the middle of the day in high heat and humidity, with Novak Djokovic arguing that matches should start later in the day.
The International Tennis Federation, which organises the event, had kept matches beginning at 11am but from Thursday that will change to 3pm.
The ITF said in a statement: "In the interests of player health and welfare and following extensive consultation, the ITF has announced a change of schedule due to the increasing heat and humidity currently being experienced in Tokyo.
"The decision to start matches at 3pm from Thursday is possible due to the outcomes of today’s matches across the five competitions being staged and the size of player field, and is designed to further safeguard player health.
"It has been made following consultation with the IOC, Tokyo 2020, Olympic Broadcast Services as well as the Olympic Tennis Event players, referee, medical experts and other key stakeholders."
Djokovic welcomed the change, saying: "It was nice news to receive. In my opinion, it should have been done a few days earlier. But it is what it is.
"It’s very good because you don’t want to see situations like what we saw today with Paula Badosa. I’ve played tennis now professionally for 20 years and I’ve never faced this kind of conditions in my entire life on a consecutive daily basis."
Medvedev lost the second set against Fognini but recovered to win 6-2 3-6 6-2 and reach the quarter-finals.
He said later: "Even from the first set, I didn’t feel good enough with my breathing. I felt like my diaphragm had blocked. I think it was the most humid day we have had so far.
"Then, in the second set, I just had darkness in my eyes. Between every point I didn’t know what to do to feel better. I was bending over and I couldn’t get my breathing together. I was ready to just fall down on the court.
"It helped that I won the first set, so I knew there was a 10-minute break. So I went under the cold, freezing shower. When you have such a change of temperature and go out on the hot court, you can fully cramp and it finishes the match for you, or you feel better. I was lucky I felt better."
Medvedev took exception to being asked about whether he felt a stigma of cheating following the ban on athletes competing for Russia, calling for the journalist who asked the question to be barred from the Olympics.