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.
"I am very disappointed that I am unable to compete this year in Miami," Azarenka said. "This is one of my favorite events and the Miami fans are always supportive.
"I have been working very hard to rehab my ankle this week but unfortunately it is still not fit to play. I look forward to returning next year."
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.
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."