British number one Elena Baltacha reached the second round of the US Open after American wildcard Jamie Hampton retired in dramatic circumstances.
Baltacha was on the verge of victory at 2-6 6-2 5-1 on Court Seven when Hampton, who had earlier called the trainer, collapsed at the baseline.
She was quickly attended to by officials and medical staff, with Baltacha also helping the American.
Fellow Briton Anne Keothavong lost 7-5 6-3 to Chanelle Scheepers.
Baltacha will next play 2004 champion and 15th seed Svetlana Kuznetsova.
The Scot, ranked 58th in the world, beat Hampton at the Australian Open in January after dropping the first set and had to dig deep again in New York.
But she became the second British woman through to the last 64, joining Laura Robson, as she outlasted world number 129 Hampton.
After breaking serve in the opening game, Baltacha saw the first set slip away in 33 minutes before going on to dominate.
Hampton required treatment from the trainer in the third set and could not keep pace with Baltacha, who got the double-break at 4-1 and was seemingly about to close out the match when the end came unexpectedly after two hours and four minutes.
"I remember when I went through that in Doha a couple years ago, when you start with a calf cramp and then it kind of works its way up," said Baltacha.
"Before you know it, it's reaching your abdominal area and then it's going into a full body cramp. She couldn't even get up.
"I thought it was quite nasty, actually, the umpire said a code violation for time. Once the umpire said that I managed to get down there and help her."
The Briton added: "I've just seen her. She's walking around. It's good that she's fine. The trainer is walking around with her just to make sure, because I think full body cramps are dangerous."
Keothavong, ranked 100, suffered a disappointing defeat against South Africa's world number 81, Scheepers, after making a bright start to the match.
From 5-2 up the Londoner lost seven straight games, failing to convert a set point on serve in game nine, and missing out on three break points at 0-40 early in the second set.
"I put myself into a really strong position in the first set, having had the set point, but she was really solid, she didn't give me anything and she just didn't miss," Keothavong told BBC Radio 5 live.
"I guess I've taken quite a few knocks recently but tennis right now is not like a life or death situation. It's incredibly frustrating but I pride myself on my hard work, and it doesn't stop just because I lost a tennis match."