DOHA, Qatar -- Simona Halep reached the Qatar Open semifinals with a dominating win, then promptly withdrew with a right foot injury on Friday.
Muguruza beat Caroline Garcia of France 3-6, 6-1, 6-4 in the quarterfinals.
"I am surprised that I could play three matches and win them," said Halep, the 2014 champion. "I felt pain every day, so it was not easy to manage it.
"I didn't have much time to recover before this tournament, but today was too much, and I decided to stop.
"The MRI shows I have fluid and also tendinitis at the fourth toe, so I have to take care of it and think about my health first."
Halep also withdrew from next week's Dubai Open, another tournament she has won, and said she did not expect to be back on court for more than two weeks, until Indian Wells.
Halep relinquished No. 1 after losing to Caroline Wozniacki in the Australian Open final.
Information from The Associated Press was used in this report.