"I am disappointed to withdraw from the Miami Open due to a left knee injury," Williams said in a statement, per the New York Times' Ben Rothenberg. "It was an amazing experience to play at the Hard Rock Stadium this year and would like to thank the Miami Open for putting on an amazing event. I hope to be back next year to play at this one-of-a-kind tournament in front of the incredible fans here in Miami."
Williams defeated Rebecca Peterson in the second round of the tournament and was set to play 18th-seeded Qiang Wang in the third round Sunday.
The Miami Open was her second tournament since losing to Karolina Pliskova in the quarterfinals of the Australian Open in January. Williams also competed in the BNP Paribas Open earlier this month, retiring in the second set of her match with Garbine Muguruza. The 23-time Grand Slam champion cited an illness as her reason for withdrawing.
In her three-set victory over Peterson, Williams acknowledged she wasn't playing up to her best.
"What was a little frustrating today is I know I can play so much better, but I just wasn't able to produce it today," she said, per the WTA's official site. "But that's the beauty of fighting, getting through the matches where you're not playing your best, then coming out the next time and just doing better."
Last year, Williams competed in the Miami Open and didn't return until the French Open. Depending on the status of her knee injury, the 37-year-old would likely at least want to play in the Madrid Open, which starts May 6.
That tournament is one of the last major stops before Roland Garros.