"I feel much better now than I did at 2 a.m. this morning," said the 14th seed from Thornhill, Ont., who was scheduled to face American Sam Querrey.
"I got sick over the last two days with strep throat. I felt really bad the last two nights. I wasn't able to sleep with the high fever. I had discomfort and pain all over the body. I only decided not to play five minutes ago, I was trying to play it out as long as possible. But I feel completely drained.
It was the second straight year that hard luck has struck the 22-year-old in Miami. In 2012, he was unable to play Andy Murray in the third round due to an ankle injury.
"It's just bad luck, nothing more," said a pale Raonic, who said he had a fever prior to playing his previous match on Saturday against Frenchman Guillaume Rufin. "Between a twisted ankle and illness, I hope that's it for bad luck in Miami.
"This will pass and I'm sure I'll be fine for Davis Cup.