NEW YORK—John Isner firmly denies that the cause of Jack Sock's retirement against Ruben Bemelmans was due to the American's level of fitness. On a hot and humid day on Thursday, the 22-year-old retired in the third game of the fourth set due to cramping, while leading two sets to one.

“It's tough to see. You see it on TV and you're helpless at that point, completely,” Isner said. “He sweats more than anyone I've ever seen. It's not a fitness thing. I think that's a big, big misconception. He's in very good shape. He can play 50-ball rallies if he wants to. But he sweats a lot. He loses a lot when he's sweating.”

Isner also played on Thursday and won his match in straight sets.

“It's all about, in my opinion, putting the right things in your body beforehand,” Isner said. "He's in very good shape. It's not a fitness thing, if people are saying that. His body was at a deficit of whatever it is, sodium, magnesium, potassium. Whatever it is, in these humid conditions, you have to put all that in your body."

The top-ranked American would have liked Sock's chances in different conditions, saying, "It's a huge bummer. No offense to his opponent today, but if Jack's body held up, he would have won the match. Huge bummer, especially at his home Grand Slam. He obviously was playing well, too, up until that point. Good thing for Jack is he's very young. He's very, very good. So he's going to have a lot more cracks at this tournament, that's for sure.”