British No.2 Heather Watson retired from her first round match at the TEB BNP Paribas Tennis Championship suffering from exhaustion.
The 28-year-old, who was seeded five in Istanbul, called for assistance after the fifth game of the first set against Spain's Sara Sorribes Tormo and reluctantly called it a day following a medical time-out.
A spokesman for Watson said: "Physios said Heather became exhausted from the recent trips (and) presented low blood sugar."
Watson, who was beaten by compatriot Johanna Konta in the first round at the US Open last week, was bidding to avoid a fifth successive defeat since winning the Mexican Open in February before lockdown.
She had got off to a poor start when she was broken in the opening game, and then had to survive two break points in the third before ending consecutive rallies with delicate drop shots to fight back and open her account.
The Briton levelled at 2-2 after setting up break point with an angled backhand volley, but surrendered parity immediately after netting another attempted drop shot with the umpire having summoned medical help.
She was treated at courtside, where she had her temperature and blood pressure checked while ice packs were used to cool her down before she took the decision to withdraw.
Jennifer Brady, meanwhile, cruised into her first Grand Slam semi-final at the US Open with a comfortable straight-sets win over Yulia Putintseva.
The 28th-seeded American never surrendered her advantage after charging into an immediate double break, and needed just over an hour to wrap up a 6-3 6-2 win.
Brady, who beat former champion Angelique Kerber in the previous round, is yet to drop a set and will play either fourth seed Naomi Osaka or compatriot Shelby Rogers in the last-four.
Brady, who was beaten in both her previous meetings with the Kazakh, said: "I felt like today I was a lot more aggressive off the first ball. I came out with nerves, obviously, and I think she did too because it's our first quarter-final, but I just tried to treat it like any other match.
"I was happy with the way I started and I was able to keep that momentum and build off that. I'm thrilled to be in the semi-finals of the US Open."
The 25-year-old Brady had shown signs of promising form when the sport resumed last month and she won her first tour title in Kentucky.
And her strong, front-foot game was too much for the awkward Putintseva, who cut a frustrated figure.