Novak Djokovic threw up, Alexander Zverev poured sweat out of his shoe and Emma Raducanu quit her match with dizziness -- and the bad news for the world's top tennis players is that more baking weather is to come in China this week.

Denmark's Holger Rune called the temperatures of more than 30C and humidity soaring past 80 percent at the Shanghai Masters "brutal".

"Do you want a player to die on court?" Rune was heard to ask this week in the fierce conditions.

The 24-time Grand Slam champion Djokovic labelled it "very challenging physically" on Tuesday as he battled the heat and an ankle injury scare to reach the quarter-finals.

The ailing 38-year-old crouched over frequently in between rallies against Spain's Jaume Munar and vomited on court.

After one unforced error, Djokovic dropped to the court and remained splayed out as a medic rushed over, before regathering himself to win in three battling sets.

The draw has opened up for the Serb after world number one Carlos Alcaraz withdrew before the tournament to rest and defending champion Jannik Sinner retired from his match on Sunday with cramp.

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Novak Djokovic wipes his face as he receives medical attention / © AFP

Zverev, ranked three in the world, was beaten on Monday having had to pause during his defeat to change his shoes because sweat was pouring out of them.

Spectators at the centre court wafted fans and wore cool packs on their foreheads to counter the suffocating humidity.

The main court in Shanghai has a roof but it remains open and would only be closed if it rains -- there is no prospect of that for the remainder of the tournament.

Rune called Tuesday for the men's governing body the ATP to introduce a heat rule at events such as the Shanghai Masters.

The ATP said in a statement that player safety was its top priority and it is considering such a policy.

Currently, decisions affecting play to do with weather conditions, including heat, "lie with the on-site ATP Supervisor, in coordination with on-site medical teams and local authorities", it said.

Conditions felt marginally less oppressive on Wednesday in Shanghai but forecasts were still for highs of 29C.

Temperatures are set to rise again in the coming days and peak on Sunday, the day of the final, at an estimated 32-33C.

France's Arthur Rinderknech, who reached the quarter-finals on Wednesday, said it was not only the players that were suffering.

"It was as hard for the ball boys, for the umpire, for the fans, everybody was always like this -- blowing air to the face because it was really hot," he said.

- 'Rather than just die' -

The conditions are equally punishing in Wuhan, central China, where all the top women's players are competing.

Temperatures are above average for this time of year in both cities.

Scientists have consistently warned that human-driven climate change is resulting in more frequent and intense weather events worldwide.

The difference between Wuhan and Shanghai is that the WTA has a heat policy.

At the WTA Wuhan Open on Tuesday, former Grand Slam champions Emma Raducanu and Jelena Ostapenko both retired from their matches.

Raducanu had her blood pressure and other vitals checked before retiring with dizziness from her first-round match.

World number two Iga Swiatek called on organisers to take player safety into account when scheduling matches on the outside courts, where there is no roof.

"On centre court I think it's a little bit cooler with the air conditioning and everything," she said.

"But I hope the other matches will be scheduled at a time where girls can compete, rather than just die on the court."

Temperatures above 30C forced organisers to put the WTA's heat rule into effect on Monday, where play was suspended on the outside courts.

The heat rule was also in effect for part of Tuesday.

The policy allows players to take a 10-minute break between the second and third sets, and means the tournament can partially or fully close the centre-court roof to protect players.

The rule looks set to come into play repeatedly this week in Wuhan, where highs above 30C are forecast all week including for Sunday's final.