Given a spot in the draw because Azarenka pulled out, Davis took advantage of shaky play by fellow Floridian Madison
Keys in the decisive tiebreaker to win their second-round match 6-1, 5-7, 7-6 (7).
Davis trailed 6-3 in the tiebreaker and then saved three match consecutive points, all on unforced errors by Keys.
This was Davis’ first victory at Key Biscayne, and the 19-year-old barely made the tournament. She learned at 10 a.m. that Azarenka had quit, and an hour later was playing on the stadium court.
“I didn’t care if I won or lost,” she said. “I just was so grateful for the opportunity to play.”
Like Keys, American Jamie Hampton held a match point but lost to No. 20-seeded Carla Suarez Navarro 5-7, 7-5, 7-6 (3).
In men’s play, No. 3 David Ferrer advanced by walkover when qualifier Dmitry Tursunov withdrew with acute gastroenteritis.
Wild card James Blake, playing at Key Biscayne for the 12th time, beat No. 24-seeded Julien Benneteau 6-2, 6-3.
Azarenka withdrew before her quarterfinal last week at Indian Wells because of inflammation and tendinitis in her ankle.
She returned to the practice court Wednesday, and the next day her injury was worse, she said.
She decided to withdraw after trying to practice Friday. Now she figures she needs another couple of weeks to recover.
Ranked No. 3, Azarenka is 17-0 this year with two titles but has withdrawn from three tournaments, including the Australian Open before the semifinals because of a right toe injury.
Once the No. 81-ranked Davis was granted a spot in the draw, she worked hard to keep it. In a matchup between two teenagers from nearby Boca Raton, she and Keys played for more than 2½ hours before reaching the tiebreaker.
Keys, an 18-year-old wild card, entered the tournament with a career-best ranking of No. 76. But when she served for the match at 6-3 in the tiebreaker, her inexperience showed.
She dumped groundstrokes into the net on consecutive points, then pushed a forehand wide for 6-all, and at 7-all she double-faulted. When Keys sailed a forehand long on the final point, she broke her racket by slamming it to the concrete.
“This will sting for a couple of days,” she said. “The whole match bugs me. Early on I was going for too much. Then in the tiebreaker I was remember what happened earlier, and that made me more nervous.”
Steadier from the baseline but a head shorter than Keys, the 5-foot-2 Davis won despite trailing 10-0 in aces and 43-10 in winners.
“I just relied on my fighting instincts to pull me through,” she said.
Davis said she and Keys have known each other for several years. They practice together often and are good friends.
“We’re always so competitive, and the score is always really, really close,” Davis said. “It’s the smallest things that make a difference.”