A scorching second day at the US Open has brought the subject of the players' safety into focus, as temperatures soared towards 40 degrees at flushing meadows.

Players scrambled for relief from the heat, while some were forced to retire from the overheated conditions. Temperatures soared to 38 degrees by mid-afternoon, made worse by suffocating humidity.

While the WTA Tour has a heat policy in its rule books, the men's ATP Tour doesn't officially have a policy to address heat situations like this, forcing the United States Tennis Association to take matters into their own hands.

The USTA introduced the heat policy allowing men to take a 10-minute break between the third and fourth sets.

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The women's WTA Heat Rule has been in place since 1992 and is used throughout the year at all WTA tournaments.

Former world No. 1 Novak Djokovic was one of the players who took advantage of the much-needed respite in his four-set win over Márton Fucsovics.

Djokovic admitted that he wasn't having his best day and that he – like everyone else – was struggling with the heat.

"I'm actually praying that the next moment I get to feel better because I definitely wasn't feeling great for most of the three sets. But you have these kind of days, you have these kind of matches," he said after the match.

"I'm not the only one. A lot of players struggled today. You just have to deal with it. We played a three hour match. Not easy but it is what it is."

However, some players like Argentinian Leonardo Mayer were forced retire because of the heat.

"I think we should no longer play five sets," Mayer said in Spanish, according to an ESPN report.

"That's my opinion, I think that's the past. They won't stop until someone dies. It's incredible, matches become ugly."

No. 5 ranked Petra Kvitova said she was lucky to have finished her match before the heat started to ramp up.

"I think I was pretty lucky to play first on: It was pretty difficult conditions," she said after her straight-sets win.

"It got hotter and hotter - and tougher. The ice towels helped a lot, of course. When you're playing, you don't think about it. When you stop for a while, you feel the heat, from the ground as well."

The issue of playing in sweltering temperatures in tennis was also a hotly debated in this year's Australian Open in January, where temperatures also soared close to 40C.

Dr Liz Hanna of the Climate Change Institute at the Australian National University and the Climate and Health Alliance told news.com.au earlier in the year that "someone could die" if players pushed their bodies too far in extreme conditions.

"(The temperatures players face) are a lot hotter than recorded and we know with tennis players - particularly when they're playing at their full capacity - their muscles are generating an enormous amount of additional heat," she told news.com.au.

"They're at very high risk of overheating and that is potentially lethal. The worst case scenario is that somebody would succumb to severe injury."

Several fans took to social media to express their outrage at the fact that players were forced to play in the conditions at the US Open.