Garbine Muguruza advanced to the last 16 of the Indian Wells tournament on Sunday after Serena Williams retired through illness. The 20th seed was leading 6-3 1-0 when the 23 time Grand Slam champion quit. Williams, 37, had lost six consecutive games before conceding.
"Before the match, I did not feel great and then it just got worse with every second," said Williams. "It was extreme dizziness and extreme fatigue.
"When I was up 3-1 in the first set, it might have looked like I started well but I was not feeling at all well physically."
Williams, who was seeded 10th, added: "I will focus on getting better and start preparing for the Miami tournament."
Muguruza will next play Kiki Bertens from the Netherlands after the seventh seed saw off Britain's Johanna Konta 7-6, 6-4.
Elsewhere second seed Simona Halep progressed following a 7-6, 7-5 win over the Ukrainian qualifier Kateryna Kozlova.
"It was really difficult against her because I never played against her," said Halep. "It was a bit difficult to get a rhythm.
"When I was leading the match, she played without fear. I felt like she was playing much better in those games," Halep said of the world number 114.
"It was difficult for me to close those sets but I'm actually proud that I could do that," she added.
Halep next faces the unseeded Czech Marketa Vondrousova who got past the 22nd seed Jelena Ostapenko in three sets.
