Novak Djokovic made routine progress into the second round of the men's singles Olympic tournament on Saturday before joining his fellow players in calling on Tokyo 2020 organisers to schedule matches later in the day to counter the heat and humidity.
Serbia's Djokovic, the world No 1, produced a 6-2, 6-2 win over Bolivia's Hugo Dellien in the first round at Ariake Tennis Park and will next face Jan Lennard Struff of Germany.
Should Djokovic leave Tokyo with the gold medal, he will keep alive his bid to become the first man to win the golden Grand Slam of all four majors and the Olympic title in the same season - with just the US Open remaining.
After his win over Dellien, he questioned why matches were being staged during the day with temperatures reaching the mid-30s degrees Celsius and humidity around 80 per cent.
"I don't understand why they don't start matches at, say, 3pm," Djokovic, 34, said. "We still have seven hours to play. They have lights on all the courts.
"You feel you have weights on your shoulders because there's so much heat and humidity and stagnated air.
"You don't feel yourself, you feel slow with your legs. It's not the first time we get to experience tough conditions. I spoke to a couple of guys in the locker room and all of them said this is the toughest that they have experienced day to day.
"I don't really get why the ITF [International Tennis Federation] doesn't want to move the matches."
Djokovic's comments came after world No 2 Daniil Medvedev - representing the Russian Olympic Committee - shared similar concerns following his 6-4, 7-6 victory against Alexander Bublik of Kazakhstan.
"I don't think they are going to change anything in the middle of the tournament, but that's what can be done," he said. "I think the fact that we have only one minute between the changeovers is a joke ... It should be 1:30 like it is in other tournaments."
Medvedev's compatriot Anastasia Pavlyuchenkova, who is competing in the women's singles, had to take a medical timeout during her first round match against Italy's Sarra Erani due to dizziness brought on by dehydration.
Pavlyuchenkova, 30, regularly padded herself with ice packs during the changeovers and was seen struggling to get the mobile air conditioner tube working properly.
"I feel better but I have a massive headache now, so that's why I was trying to try to cool down after the match and rest, relax little bit," said Pavlyuchenkova, who despite her difficulties with the heat defeated Errani 6-0, 6-1.
In response to the players' concerns, the ITF issued a statement stating: "Great consideration has been given to the 11am or a later start. The decision to retain 11am was made based on data, a nine-day event and to accommodate factors such as local authority restrictions due to Covid-19 and the unpredictability of the weather.
"Extreme conditions will always put pressure on a schedule and make an optimum schedule challenging. Rainfall is as much of an issue as extreme heat. An Extreme Weather Policy is in place. Tokyo 2020 competition officials have set procedures to monitor the heat stress index which determine whether play should be modified and have implemented these procedures today."
Great Britain's two-time defending Olympic champion Andy Murray gets his singles campaign underway on Sunday when he faces Canada's Felix Auger Aliassime.
Japan's world No 2 Naomi Osaka, who lit the Olympic torch at Friday's opening ceremony, also begins her bid for gold on Sunday against China's Saisai Zheng.