Three-time Miami champion Venus Williams withdrew from the WTA and ATP Masters Miami hardcourt tournament on Saturday with a lower back injury.
The US 19th seed was to have faced 16th-seeded compatriot Sloane Stephens in a third-round match at the $8.5 million event.
Williams had opened by defeating Japan's Kimiko Date-Krum 7-6 (7/3), 3-6, 6-4 but said she could not be sure if the tough match played a role in forcing her out of the tournament.
"Yesterday I was having some pain, and then just wanted to see how I felt in the warmup -- just not able to play today," Williams said. "It's really disappointing. This is just the kind of tournament you want to play well at.
"But I have faced disappointments in my life and my career. It's not the first, probably not the last, just have to get through it and stay positive."
In 2011, Williams was diagnosed with Sjogren's syndrome, an auto-immune disease that can cause fatigue.
Williams is a seven-time Grand Slam singles champion whose most recent major title came in 2008 at Wimbledon, where she is a five-time champion.
She hopes to play again next month in Charleston, South Carolina.
"I've dealt with injuries before in my whole career," she said. "I know how to hopefully recover quickly from them. I'm playing in Charleston, so that's really still my goal, to get ready to play there."
With the walkover, 20-year-old Stephens advanced to a matchup against defending champion Agnieszka Radwanska, who outlasted Slovakia's Magdalena Rybarikova 7-6 (7/5), 2-6, 6-3.
The Polish fourth seed has a chance to become the first repeat women's winner at Miami since Serena Williams in 2007-2008.