The risks of heat-stroke at one of the hottest Games on record are borne by the athletes, but also by the thousands of staff, especially at outdoor venues.
Some 30 people involved in the organisation of the Olympics have suffered heat-related illness so far but all had mild symptoms, Games director general Toshiro Muto said.
"Before the outset of the coronavirus problem, the important issue for the Tokyo Games was a response to heat illness," Muto told reporters. "We looked into all sorts of scenarios to take thorough measures. I believe our steps have been working well so far."
Bruno Schmidt, Brazilian 2016 gold medallist in beach volleyball, said the Games were hotter and muggier than he had expected. "The first two weeks here are one of the hottest that I had in my life, believe it or not."
But South Africa's Wayde van Niekerk, 400-metre world record holder and 2016 gold-medallist, said that while it would be nice if the humidity could be dialled down, "every competitor needs to deal with it and we take it in our stride".
World Athletics spokesperson Nicole Jeffery said the timetable has been designed to hold endurance events in the evening when it is cooler.
The packed evening agenda includes the marquee men's 100-metre final.
"All athletes are provided with water and ice and the medical team is observing them closely to make sure no-one is showing signs of heat stress," Jeffery said.
Jeffery added "cold water immersion facilities" were available for anyone suffering from heat.
The morning athletics programme saw China's Gong Lijiao win the women's shot put gold as qualifying rounds in the women's hammer throw, long jump and men's 400 metres took place.
The Tokyo Games, from July 23 to Aug. 8, coincide with the year's hottest weather in Tokyo where the temperature can rise to 35 degree Celsius (95°F) or more. The 1964 Tokyo Summer Olympics were held in October.
A study last year by a Games adviser, analysing data back to 1984, found that Tokyo had the highest average temperature and precipitation of any host city for the period the Olympics were held. Tokyo's five hottest days since 1964 fell in or around the period of this year's Games.
In 2013, the Tokyo bid committee promised "many days of mild and sunny weather", providing "an ideal climate for athletes to perform at their best", in defending the timing, which was driven by global broadcast schedules, when the global sports calendar is otherwise light.
Battling stifling temperatures on Wednesday, tennis world No. 2 Daniil Medvedev warned officials a player "can die" in the heat. The sports' governing body later agreed to delay match start times in response to similar complaints by other players.
Hockey players, sweltering on an unshaded pitch, were given double the usual number of two-minute breaks.
At the equestrian cross-country course, France's Karim Florent Laghouag donned an ice vest, a wet towel and bags of ice around his neck when speaking to journalists after his ride.
"This is so good," he said, pointing at the vest as temperatures climbed well over 30 degrees Celsius.
Organisers have deployed a host of tools - from mist-spraying stations to cooling vests to AI gadgets that warn of heat-stroke risk - to beat the heat while handing out salt tablets and ice cream to volunteers.
Tokyo has also used roads that reflect heat or pavements that absorb water and the organisers moved the marathon and race-walk to the cooler northern city of Sapporo.
(Reporting by Amy Tennery and Daniel Leussink; Additional reporting by Mari Saito, Shadia Nasralla, Rohith Nair, Omar Mohammed and Kiyoshi Takenaka; Writing by Antoni Slodkowski and William Mallard; Editing by Himani Sarkar and Lincoln Feast.) ((Amy.Tennery@thomsonreuters.com; 646-223-6343;))