World number one Djokovic, who beat Bolivia’s Hugo Deillien in straight sets in their first-round encounter, called for matches to be pushed back to later in the day.
The Serb said: “I don't understand why they don't start matches at, say, 3pm. 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."
The 34-year-old said the other players he had spoken to had agreed that the conditions were the “toughest that they had experienced”.
He continued: “You don't feel yourself, you feel slow with your legs. It's not the first time we get to experience tough conditions. I don't really get why ITF [International Tennis Federation] doesn't want to move the matches."
Tennis’ world governing body agreed in a statement that players’ health was of the utmost importance.
A statement read: "Great consideration has been given to the 11am or 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."
World number two Medvedev also urged a rethink, suggesting matches start at 6pm. The 25-year-old said it was a “joke” that players were permitted just one minute at changeovers rather than the standard 90 seconds.
The Russian won his first-round match against Kazakhstan’s Alexander Bublik 6-4 7-6 (10-8) in heats of up to 32C at the Ariake Tennis Park, later describing the conditions as “some of the worst” he had ever experienced.
Medvedev’s countrywoman Anastasia Pavlyuchenkova was forced to take a medical time-out in her match against Italy’s Sara Errani after the 30-year-old began feeling dizzy in the sweltering heat.
She used ice bags to cool down before finishing a straight sets win, saying afterwards: “I feel better but I have a massive headache now… that's why I was trying to try to cool down after the match.”
The Games only officially started on Friday but already the weather has become a huge talking point.
Russian archer Svetlana Gomboeva fainted in extreme heat during her qualifying competition and the rowing scheduled was altered amid concerns over a typhoon.
Scotland’s Seonaid McIntosh also highlighted the high temperatures as she competed in the 10m air rifle event.
The last time the Olympics were held in Tokyo, in 1964, organisers pushed the Games back to the autumn to avoid the summer’s high temperatures.
The marathon and race-walking events have already been moved to Sapporo, where the weather is cooler, but experts have already warned that heat and humidity in the Japanese capital could prove dangerous for competitors.
Japan’s Environment Agency has already issued heatstroke alerts and warned people not to exercise outdoors.