She lost the next seven games and was trailing 3-6 0-1 when she called it a day.
Williams was reported as saying on the tournament’s official website, “Before the match, I did not feel great, and then it just got worse with every second; extreme dizziness and extreme fatigue,”
She also 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.”
A Tweet from tournament organizers later said Williams was suffering from a “viral illness.”
She was in fine form as she defeated former world No.1 Victoria Azarenka in the second round on Saturday. Williams was competing in her first tournament since exiting the Australian Open at the quarter-final stage in January.