“We have never been so popular,” she said of the tournament’s medical staff.

The United States Tennis Association has twice needed to issue news releases clarifying its rules regarding changes of attire — yes, women, you can now change your tops on court without being penalized — because there have been more changes of attire than ever.

John Isner said he went through 11 shirts in his three-and-a-half-hour quarterfinal loss to Juan Martín del Potro on Tuesday. He also kept a rotation of many upside-down hats next to his chair, switching into whichever one was driest on changeovers, and asked for sawdust to improve his grip on his racket. He estimated he lost between 8 and 10 pounds during the match.

The heat wave broke briefly last weekend — and cooler days are ahead this weekend — but conditions inside Arthur Ashe Stadium during the second week of the tournament have been especially draining.

That was never clearer than on Monday night, when second-seeded Roger Federer, whose rivals and fans joke about never having seen him sweat, exited the tournament in a soppy puddle of surrender.

Federer complained that he “couldn’t get air” and said he was sweating so much that the spare tennis balls in his pockets were becoming dampened, which made the slow conditions even slower.

“At some point, I was just happy that the match was over, I guess,” Federer admitted after falling to Millman in the fourth round.