For two sets, 28th-seeded American Jack Sock was cruising against Belgian Ruben Bemelmans in a second-round U.S. Open match Thursday on the Grandstand Court. Then, the heat-and-humidity monster that has plagued the early rounds claimed another victim.

Sock slowed in a third-set loss and then ground to a halt while trailing 2-1 in the fourth set when he retired because of left thigh cramps and heat exhaustion. Even Bemelmans expressed sympathy for the overheated Sock.

"It's not a pretty sight," Bemelmans said. "I know how it feels when you are cramping and dizzy. I experienced it in Davis Cup. I was feeling sorry for him. He was playing well.

"I was trying something different in the third set. It was working, but maybe he wasn't moving as well as he was in the first two sets. It's bad luck for him that he had to retire."

Sock's retirement was the 14th in four days, including 12 on the men's side, where they play best-of-five sets. When the end came, Sock tried massaging the cramp out of his thigh but had to lie down to receive treatment, including ice packs, before he was helped off the court.

Several hours after the match, Sock issued a statement in which he said, "Playing in the U.S. Open is the biggest and most important moment of the season for me, so having to retire from my match today is extremely disappointing."

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Bemelmans said he didn't think Sock actually fainted, as some suggested, but when he approached Sock, who was seated on the court, and tried to shake hands, the listless American could barely lift his hand.

"You could see he was all white and had no more energy in his body," Bemelmans said. "He was done."

Temperatures peaked at 92 degrees with humidity at 42 percent about the time Sock gave way and laid down on the court, according to the National Weather Service, but Bemelmans, who said he was adding salt to his water and food to stay hydrated, didn't find it too uncomfortable.

"I was still moving good on the court and feeling fit," Bemelmans said.

Sock's setback left American men with a 2-3 record for the day. Rajeev Ram lost, 7-6 (10), 6-4, 3-6, 6-3, to No. 22 Viktor Troicki of Serbia, and Austin Krajicek fell to No. 15 South African Kevin Anderson, 6-3, 6-4, 6-2.

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But Donald Young followed his first-round upset of No. 11 Gilles Simon with a 3-6, 6-4, 6-4, 6-2 win over Great Britain's Aljaz Bedene, and No. 13 John Isner, the top-seeded American, won, 6-3, 6-4, 6-4, over Russian Mikhail Youzhny, who won their two previous meetings.