Grasping for a rubber tube blowing cool air next to his seat on changeovers.

Two medical timeouts and one visit from a trainer.

Daniil Medvedev was struggling so much with the suffocating heat and humidity at the Ariake Tennis Park on Wednesday that at one point the chair umpire, Carlos Ramos, asked him if he could continue playing.

"I can finish the match but I can die," Medvedev replied. "If I die, are you going to be responsible?"

Afterward, Medvedev said he felt "darkness" in his eyes.

"I didn't know what to do to feel better," the ROC player said.

"I was ready to just fall down on the court."

Still, the second-seeded Medvedev produced a 6-2 3-6 6-2 win over Fabio Fognini of Italy to reach the quarter-finals at the Tokyo Olympics.

Spanish player Paula Badosa was less fortunate.

She left the court in a wheelchair after retiring from her quarter-final match against Marketa Vondrousova because of illness.

Vondrousova, the Czech player who eliminated Naomi Osaka a day earlier, had won the first set 6-3 and is now into the semi-finals medal round.

The temperature rose to 31 degrees Celsius on Wednesday but the heat index made it feel like 37 degrees with humidity above 70 per cent.

It raised questions over why organisers did not grant requests earlier in the tournament from Medvedev and other players - including top-ranked Novak Djokovic - to move all of the tennis matches at the Games to the evening.

On Wednesday evening the International Tennis Federation announced that from Thursday matches would start at the later time of 3pm, it noted this was possible due to reduced fields after several days play and was "to further safeguard player health".

Medvedev received medical treatment and had his chest massaged while leading 5-2 in the opening set but then held his serve in the next game to close it out.

He then took another medical timeout while trailing 4-3 in the second.

"I felt like my diaphragm has blocked. I couldn't breathe properly," Medvedev said.

Medvedev was in better form in the final set, breaking the Italian's serve twice to seal the victory.

He will next face sixth-seeded Pablo Carreno Busta of Spain for a spot in the medal rounds.

Carreno Busta beat Dominik Koepfer of Germany 7-6 (9-7) 6-3.

Meanwhile, Greece's French Open finalist Stefanos Tsitsipas was beaten 2-6 7-6 (7-4) 6-2 by France's Ugo Humbert after taking a nasty tumble at set point in the second set tiebreak.

Japan's Kei Nishikori, the host country's last hope for a singles medal, moved into the quarter-finals with a 7-6 (7-0) 6-0 win over Ilya Ivashka of Belarus.

Meanwhile in the women's singles, French Open finalist Anastasia Pavlyuchenkova lost 6-0 3-6 6-3 in the quarter-finals to Belinda Bencic of Switzerland.

Bencic will play either Spain's Garbine Muguruza or Elena Rybakina for a spot in the gold medal match.

Ukraine's world No.6 Elina Svitolina - the fourth and highest surviving seed - also advanced to the semi-finals with a 6-4 6-4 win over Camila Giorgi of Italy.

The newly-wed Svitolina - who was watched by husband and French tennis player Gael Monfils - next plays Vondrousova.

Australian Associated Press