Kvitova, the former Wimbledon champion, was on the verge of becoming world number one last year. But her decline in results continued with a lacklustre 6-3 6-0 loss to world number 81 Alison Riske in the third round at Flushing Meadows.
Kvitova, currently ranked 10th in the world, had her blood pressure taken during a medical time out in the second set of a defeat that took one hour and five minutes.
"Unfortunately I was lying in the bed yesterday and I had a fever, so I didn't come to the site yesterday," she said.
"My body didn't help me today. I tried to play, tried to fight. But my body wouldn't let me fight.
"That's life. It's not only in the tennis. It's something I hope can help me and make me a little stronger."
Kvitova's early departure replicated unexpected losses at the three other Majors this year.
She was beaten by Britain's Laura Robson in round two at the Australian Open, American Jamie Hampton in round three of the French Open and upset by Belgium's Kirsten Flipkens in the Wimbledon quarter-finals.
She was also hampered by a virus at The All England Club.
"I think it's the same," she said. "I had a blood test to see if it was bacteria and virus, and it was virus. I mean, I didn't have any sore throat or anything like that. I had just a very high fever.
"It's unlucky at a Grand Slam. I will take it for this year and next year will be better."
Kvitova was a heavy favourite against the 23-year-old American after beating her recently in New Haven. But it quickly became apparent that Kvitova was ill.
"I couldn't play really long rallies, like more than three shots in the rallies," she said.
"So I try to play my aggressive game. First or second shots, to have a winner. I am disappointed."
She was scheduled to have more blood tests before leaving New York.
Riske was in tears during her on-court interview.
"I believe I belong here," she said.
Riske revealed the faith she placed in a small blanket given to her on the day she was born.
"The blankie story is out," she said after her first victory against a top-10 player.
"I'm used to it now. I can't deny it now. It's getting smaller by the week. It can fit in the palm of my hand.
"My siblings always used to hide it. Used to make me so mad."
The American wildcard said her good luck charm would stay in her hotel room in Manhattan during her fourth round appearance on Monday.
"It's been with me since the second I was born," she said. "It's been around the world. It started out forest green and now it's like mint green. I don't know if it's a colour now."