Serena Williams retired from her third-round match at Indian Wells while trailing Garbine Muguruza 6-3 1-0.
The 37-year-old won the first three games of the match but called for the trainer after losing six in succession to the Spaniard.
She took to the court for the second set, but retired after world number 20 Muguruza held in the opening game.
Williams was seeded 10th in her first tournament since a quarter-final exit at the Australian Open.
“Before the match I did not feel great and then it just got worse with every second – extreme dizziness and extreme fatigue,” the American said.
“By the score it might have looked like I started well, but I was not feeling at all well physically. I will focus on getting better and start preparing for Miami.”
Muguruza, who beat Williams in the 2016 French Open final, next faces seventh seed Kiki Bertens, who beat Britain’s Johanna Konta.
“We’ve played many times and it’s always super-tough, super-exciting,” said Muguruza, who is a former world number one like Williams.
“I wish I’m going to see her soon and [she’s] feeling better.”
World number two Simona Halep survived a strong challenge from qualifier Kateryna Kozlova to reach the fourth round.