Top-ranked Victoria Azarenka lost 10 of the first 11 games, then rallied to remain unbeaten this year by overcoming Dominika Cibulkova 1-6, 7-6 (7), 7-5 Monday in the fourth round of the Sony Ericsson Open.
Azarenka won her 26th consecutive match, a streak that began at the start of this year. Her start is the best on the women's tour since Martina Hingis went 37-0 to begin 1997.
In the second set, Azarenka fell behind 4-love. She found herself two points from defeat five times but swept the last three games and converted the first match point when Cibulkova sailed a passing shot long.
Azarenka has won her four previous tournaments this year, including the Australian Open for her first Grand Slam title. She seeks her third Key Biscayne title.
Serena Williams matched a career high with 20 aces to beat Samantha Stosur 7-5, 6-3 and advance to the quarterfinals.
"My serve was hot," Williams said. "I was like, 'That's pretty cool.'"
Williams avenged a loss to Stosur when they last met in the U.S. Open final in September.
"That wasn't in my mind," Williams said. "I just thought, 'This is a new game.'"
Also reaching the quarterfinals was No. 2 Maria Sharapova, who overcame 11 double-faults to beat Ekaterina Makarova 6-4, 7-6 (3). Sharapova won despite committing 52 unforced errors and losing her serve four times.
No. 1 Novak Djokovic advanced to the men's fourth round by beating No. 27-seeded Viktor Troicki 6-3, 6-4. Djokovic is bidding for his third Key Biscayne title and second in a row.
No. 8-seeded Mardy Fish assured he'll remain the top-ranked American man by beating No. 28 Kevin Anderson 6-4, 6-3. Fish has yet to play in the stadium this tournament and instead faced Anderson at 11 a.m. on the grandstand court, but he said that was fine because prefers an early start.
In other women's play, former No. 1 Caroline Wozniacki defeated No. 23 Yanina Wickmayer 7-6 (6), 6-0. Reigning French Open champion Li Na rallied past Sabine Lisicki 3-6, 6-4, 6-2, and No. 5 Agnieszka Radwanska defeated 18-year-old Garbine Muguruza Blanco 6-3, 6-2.
The No. 10-seeded Williams lost only six points on her first serve against the No. 6-seeded Stosur, but had to rally after being broken in the first game of each set.
Williams failed to convert her first seven break-point chances, then muscled her way back into the match. She also showed finesse, such as when she chipped a forehand drop shot that barely cleared the net for a dainty winner.
Williams is playing in her first tournament since January after being sidelined by a left ankle injury. She hasn't won a title since August, and seeks a record sixth Key Biscayne championship.
"I'm really trying so hard," she told the crowd. "My mom gave me the ultimate pep talk last night, so thanks, Mom."
Her sister Venus was to face Ana Ivanovic in the last fourth-round match Monday night. Venus is playing in her first tournament since withdrawing from the U.S. Open last August after being diagnosed with a fatigue-causing autoimmune disease.
"We both have been through a lot," Serena said. "And each match, whether we're beating top-10 player or a top-80 player, we really thoroughly enjoy it and want to do really well."