Weather was a hot topic at the US Open again as players again battled high heat and humidity as well as each other in a bid for semi-final berths.

A day after Roger Federer’s hopes of a sixth title melted away in a shock loss to 55th-ranked John Millman, the US Open’s extreme heat policy was again in effect, allowing a 10-minute break in both men’s and women’s matches that went beyond straight sets on Tuesday.

“I had a shower, lay on the table and I didn’t want to come back again,” third-seeded Argentine Juan Martin del Potro said of how he spent the break in his 6-7 (5/7), 6-3, 7-6 (7/4), 6-2 victory over American John Isner.

“It was too hot to play tennis,” Del Potro quipped.

But the combination of heat, humidity and blazing sunshine was no joke.

Junior matches were suspended for several hours when the wet bulb globe temperature measurement — which assesses the effects of heat, humidity, sunlight and wind, exceeded 32.2 C — a level that poses risk of heat stress.

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Federer, who has won 20 Grand Slam titles in an array of conditions, said he felt he “couldn’t get air” in the during his Monday night match against Millman.

“It’s one of the first times it’s happened to me … (you) just keep on sweating more and more and more and more as the match goes on.”

Federer said he thought such conditions had become more problematic on Ashe since the addition of the retractable roof led to diminished airflow in the 23,000-seat stadium.

But players have suffered on all courts — five retiring from heat-related problems in the first round to prompt organizers to implement the extreme heat policy — the first time in Grand Slam history that such a break has been offered during men’s matches.