The Romanian saved three match points at 11-10 in deciding set of the Rod Laver Arena classic, overcoming Davis after the unseeded American lost a toenail in the marathon third set.
Halep, who turned her left ankle during her first match here, managed to dart around the court with the ankle taped tightly, but will likely be concerned as to how she will recover after this endurance test.
"I just kept believing that it's not over, even if she's leading a little bit more". "My ankle is, I don't know how it is because I don't feel it anymore, but it was nice to be on court. But now I improved in that way and it makes me really happy that I change myself in a good way".
Davis was a career-high 26th in the world in May past year after winning her first WTA tournament in Auckland, before slipping to her current 76.
The third was simply out of this world.
"For sure, I'm stronger mentally, and I could resist like (that) for every moment in the match", Halep said.
Each time, Davis jumped off the deck to break back, fist-pumping to an appreciative crowd.
Halep dodged a major orange cone in her quest for her first Gand Slam title Saturday.
Halep then earned two chances to serve out the match - and both times, she faltered.
Halep moved in for the kill but the American saved five break points, and later thanked the net cord for helping her hold again at 12-12.
That's if she can walk.
"But she was also getting to a lot of balls and definitely made me play my best tennis".
"There's a lot that I think I can work on going home".
Despite serving at 46 per cent on first serves, Keys never gave the Romanian a sniff taking the opening set without facing a break point. "And then I was really excited to start the new season".
Federer entered the Australian Open previous year under similar circumstances, coming off an extended break for a knee injury, and went on to win the title.
Hsieh Su-wei, who beat Wimbledon champion Garbine Muguruza in the second round, added another seeded player to her Australian Open victory collection with a 6-2, 7-5 win over No. 26 Agnieszka Radwanska.
"At 13-all when I took the game, I said "it's my lucky number".
For top-ranked Halep, already playing with an injured left ankle, the 4-6, 6-4, 15-13 victory Saturday was a testament to her renewed self-belief. We look at failure as a negative, and, like, say, you miss a shot by an inch and you do everything right.