'Extreme dizziness, extreme fatigue': Serena Williams exits Indian Wells due to viral infection
Williams had taken a 3-0 lead in the first set before losing seven straight games, including the first game of the second set, when the obviously troubled Williams took an unscheduled break in California.
The multiple-time Grand Slam winner was breathing heavily and, after a brief discussion with supervisor Donna Kelso, Williams retired from the match, handing victory to Spain's Muguruza.
Williams was suffering from a viral infection, according to tournament officials, which was later confirmed by Williams herself.
"Before the match, I did not feel great, and then it just got worse with every second; extreme dizziness and extreme fatigue," Williams said in a statement.
"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," she said, referencing next week's Miami Open.Also on rt.com Will Smith ‘not black enough’ for role as father of Williams tennis sisters in new biopic
For her part, Muguruza says that she took no pleasure from her unconventional victory against one of the sport's all-time greats.
"I felt that maybe she wasn't feeling well," she said. "It's really a weird feeling, because I don't feel like I won the match point and (got the) 'well done, ... good match'. It was just like, 'man, we'll play next time.'"
Williams, currently ranked number ten in the world, has won a record 23 Grand Slam titles - one more than former record holder Steffi Graf.