Temperatures close to 38C (100F) in New York were made more stifling by humidity levels of over 50%.
French player Julien Benneteau, who won on day two, said matches in the middle of the day should not have been played.
“They were lucky they only had retirements,” said the world number 60.
The conditions led to tournament organisers implementing an extreme heat policy in men’s matches for the first time.
Wimbledon champion Novak Djokovic, who said he “struggled” with the heat in his win over Marton Fucsovics, added that the physical issues affecting some players were “really sad to see”.
Six players were forced to retire from matches on Tuesday, with the United States Tennis Association (USTA) saying five of the withdrawals – Italian qualifier Stefano Travaglia, Lithuanian Ricardas Berankis, Argentine Leonardo Mayer, Russia’s Mikhail Youzhny and Serb Filip Krajinovic – were “heat-related”.
Djokovic, who asked for a sick bucket to be put next to his chair in the second set, added: “There’s so much cramping going on. You don’t want to see that.
“You have to be fit, of course. I agree with that. But there are some conditions that are so extreme that, as fit as you are, you can’t just not feel it.”
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