Djokovic looked in real trouble in the steamy conditions against Marton Fucsovics and admitted to feeling sick during the nearly three hour encounter but recovered to win 6-3 3-6 6-4 6-0 on Arthur Ashe Stadium. But it's in these kind of moments you learn lessons and hopefully I'll move on a better player'.
Djokovic, 31, missed a match point in the third set before Sandgren dominated the tie-break.
The U.S. national weather service says its heat advisory for New York City will remain in place until Wednesday night.
Federer launched his quest for a sixth US Open title - and a first since 2008 - not under the blazing sun but under the floodlights on Ashe.
In the women's draw, Jelena Ostapenko sweated out a 6-4, 4-6, 7-5 win over German Andrea Petkovic to reach the second round. Obviously the last couple months have been really, really good in terms of results.
The pair, who were not allowed to speak to their coaches during the break, spent the downtime side-by-side, "naked in the ice baths", Djokovic said in a post-match broadcast interview.
The US Open is set for another sweltering day after players and fans wilted in temperatures that soared over 100 Fahrenheit (38 degrees Celsius). The tournament referee and the US Open medical team monitor the on-site conditions to determine when the policy will no longer be in effect.
Novak Djokovic engaged "survival mode" as he battled past Marton Fucsovics in brutally hot and humid conditions at the US Open.
"He refreshed and took an ice-bath".
Sandgren's power blew Djokovic out of the breaker, but he ran out of steam as Djokovic wrapped up the fourth set in 33 minutes.
"I got pissed off or something - at myself, my game or something". Should the men play best-of-three-set matches at majors, instead of best-of-five?
The WTA rule allows women to leave the court for 10 minutes between the second and third sets, and on Tuesday men were allowed to take similar breaks between the third and fourth sets. Leonardo Mayer and Ricardas Berankis also succumbed while veteran Mikhail Youzhny was consoled by opponent Marcos Baghdatis as he lay stricken on court with cramp.
"I had heat stroke", Mayer said.
"It felt worse once I went out into the air-conditioned room and then I went back".
"There's so much cramping going on", he said, noting that while it's a player's responsibility to be fit "there are some conditions that are so extreme that as fit as you are you can't just not feel it".
The Frenchwoman's serve in particular was very impressive, sending down seven aces and denying Konta a break point throughout the match.
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