The Spaniard is favourite to extend his record against the Argentinian despite having played two of the longest matches of the tournament

The United States Tennis Association on Thursday issued a reminder that supervisors “will decide whether to retire” a player perceived to be suffering from heat exhaustion.

So, if an official from the International Tennis Federation thinks Juan Martín del Potro is close to collapse (which he has often given the impression of being throughout his career) in his semi-final against Rafael Nadal on Friday, he can, after taking medical advice, tell the Argentinian he should quit.

Novak Djokovic ends John Millman's US Open run to reach semis Read more

There is the rider that “the ITF supervisor shall use great discretion before taking this action”, which nonetheless robs the athlete of his right to explore the limits of his physical resources.

It is hot here but it is not as consistently oppressive and health-threatening as in 2014 when there were a string of days when the temperature hit the Fahrenheit century barrier – prompting Andy Murray, one of the game’s great grinders, to call for the extreme heat policy that has been implemented this year.

When the kids came out to play on Thursday, they were dispatched back inside because of the heat, as were the wheelchair competitors, although the men’s doubles semi-final went ahead on the Louis Armstrong court. In that match, which lasted two hours and 46 minutes, Lukasz Kubot and Marcelo Melo beat Radu Albot and Malek Jaziri 7-6 (3), 3-6, 6-3. “We are very glad that we survived that,” Melo said courtside.

All that said, the odds of a man in a suit interrupting Del Potro’s progress in this tournament are significantly longer than Nadal doing that job for them. It will be hot enough when they go on court around 4pm local time – although cooler than midweek – and Nadal’s wicked forehand top-spin and delicious undercut backhand will draw every drop of perspiration from his 6ft 6in opponent getting up above his shoulder in reply, as well as reaching down for those slices.

Facebook Twitter Pinterest Rafael Nadal raises his hands to the sky after defeating Dominic Thiem. Photograph: Anadolu Agency/Getty Images

The Spaniard has come through the two longest matches of the championship: four hours and 49 minutes seeing off Dominic Thiem in a belting quarter-final, only days after spending four hours and 23 minutes taking care of business against Karen Khachanov. Yet in each draining contest there has been more zest in Nadal’s 32-year-old legs than those of his younger opponents.

Naomi Osaka and Kei Nishikori reach US Open semi-finals in contrasting styles Read more

If he is fit and recovered, he will be favoured by most sensible observers to extend his career record over Del Potro to 12-5, regardless of the big man’s own excellent performances here. On Tuesday Del Potro took his ace count to 66 – with only 10 double faults in five matches – when he outlasted that other beanpole with a non-stop engine, John Isner, in four sets and two tie-breaks over three and a half hours. He has hugely appreciated the two days’ rest before facing the fire of Nadal’s aggression.

In the later semi-final, Kei Nishikori will ride a wave of national sentiment, after his compatriot Naomi Osaka’s first slam semi-final, against last year’s finalist Madison Keys, on Thursday. And his record against Novak Djokovic is not great: two wins from 16 matches. However, one of those successes is significant in these circumstances.

Sign up to The Recap, our weekly email of editors’ picks.

When he made his remarkable run to the 2014 final, Nishikori had been unable to train properly for weeks because of a cyst on his foot but battered Djokovic in heat every bit as bad as the players have endured over the first 11 days of this tournament. The Serb, who started like a train, was pretty much out on his feet at the end, even though their four sets did not reach three hours. He has looked in a similar state of distress in a couple of matches here. Nishikori is in a good place to go for the prize again.

And, in another announcement by the USTA, it and its affiliates said on Thursday they are donating $350,000 to “restore tennis facilities and restart tennis programming throughout Puerto Rico”, which was so devastated by a hurricane a year ago.