Organisers acceded to player requests to change the tennis schedule after Spain's Paula Badosa suffered heatstroke and left the court in a wheelchair.
Badosa needed treatment after losing the opening set of her singles quarter-final 6-3 to Naomi Osaka's conqueror Marketa Vondrousova.
She later recovered but pulled out of her mixed doubles opener alongside Pablo Carreno Busta, with her coach Javier Marti saying: 'The conditions today were so tough to play in.
Paula Badosa is pictured in action during her quarter-final against Marketa Vondrousova
After losing the first set 6-3 the Spaniard needed treatment following the intense heat
'In her last two points, she was feeling very bad, she was serving very slow. After she asked to get treatment, she had no energy left and couldn't continue as she was feeling dizzy.'
Men's second seed Daniil Medvedev also struggled and was heard to say during his third-round match against Fabio Fognini: 'If I die, who will take responsibility?'
A number of players have complained about being forced to play in the middle of the day in high heat and humidity, with Novak Djokovic arguing that matches should start later in the day.
A shocking simulation shows how athletes competing in the Tokyo Olympics risk heatstroke, dehydration and exhaustion because of the extreme temperatures at what is set to be the hottest games on record.
Covered by a towel and carried off in a wheelchair, Badosa made a recovery but was forced to retire from her singles clash in the women's tennis singles competition
Running in 90°F (32°C) heat and 90 per cent humidity can raise their core temperature over the 'tipping point' of 102°F (39°C) where it becomes dangerous, experts say.
The International Tennis Federation, which organises the event, had kept matches beginning at 11am but from Thursday that will change to 3pm.
The ITF said in a statement: 'In the interests of player health and welfare and following extensive consultation, the ITF has announced a change of schedule due to the increasing heat and humidity currently being experienced in Tokyo.
'The decision to start matches at 3pm from Thursday is possible due to the outcomes of today's
