World No. 3 Victoria Azarenka withdrew with a right ankle injury while Spanish men's third seed David Ferrer advanced by walkover Friday at the WTA and ATP Masters Miami hardcourt event.
Reigning Australian Open champion Azarenka complained of ankle pain after a 6-3, 6-1 fourth-round victory over Urszula Radwanska in the Indian Wells quarter-finals last week before withdrawing eight days ago.
"The last two days I tried to practice, which did not get better so it just gives me a very obvious indication that it's not possible for me to play," Azarenka said.
"It was really obvious what the decision was couple days ago already. I just wanted to give my 100 percent possibility to play and today was my last test."
Ferrer moved into the third round of the $8.5 million tournament when Russian opponent Dmitry Turnsunov withdrew with a stomach bug.
He next faces Italian 32nd seed Fabio Fognini, who beat France's Michael Llodra 6-3, 6-1.
World No. 1 Novak Djokovic plays his first match later Friday against Czech Lukas Rosol while Argentina's fifth-seeded Juan Martin del Potro meets German Tobias Kamke and Russian third seed Maria Sharapova meets Canadian wild card Eugenie Bouchard.
Azarenka, who like all Miami seeds received a first-round bye, is unbeaten on court this year, a 17-0 run that includes the Australian Open and a title in Doha.
But she has twice surrendered matches by walkover, the first in January at a Brisbane semi-final against Serena Williams with an infected right toe that resulted from a pedicure.
"I feel like I'm in a good tennis shape, not physical shape right now," said Azarenka. "Even though I didn't feel very good physically, I played well in Indian Wells and still won three matches pretty good."
"Today I went on the court and I got more pain. I cannot really move. It makes me very frustrated.
"I have to be smart not to keep this coming and keep it coming like a snowball -- it's getting better and then it gets worse. I want to get rid of it, and then I can play on it and do the best job as I can pain free."
Azarenka said she has no idea when she might return, noting her next scheduled event is in early April at Monterrey, Mexico.
"I have not made my decision on that," she said. "I feel like I took a step back with my recovery trying to play here and trying to push. So it probably will go back to the two weeks maybe. Maybe less."
Azarenka was replaced in the Miami draw by 19-year-old US lucky loser Lauren Davis, who saved three match points in the deciding tiebreaker to defeat compatriot Madison Keys 6-1, 5-7, 7-6 (9/7).
Davis, ranked 81st, never expected to even have the chance to play after watching Azarenka warm up in the morning. Now she awaits either Britain's Laura Robson and France's Alize Cornet to decide a spot in the last 16.
"I'm so ecstatically happy," Davis said. "I came here this morning with like no hope left that I was going to get in, because I had seen Azarenka practicing and stuff, but I just hoped and I wished and I prayed that I would get in."
"I got a call at 10 a.m. saying I got in. I was just so happy. I didn't care if I won or lost. I just was so grateful for the opportunity to play."