MELBOURNE, AUSTRALIA - The Round of 16 for the bottom half of the draw was set on Day 5 of the Australian Open, and for No.21 Magdalena Rybarikova, No.37 Elise Mertens, and No.81 Petra Martic, their second-week spots are a reminder that careers can change in flash. 

Round of 16 - Bottom Half

Petra Martic vs. Elise Mertens
[Q] Denisa Allertova vs. [4] Elina Svitolina
[32] Anett Kontaveit vs. Carla Suárez Navarro
[19] Magdalena Rybarikova vs. [2] Caroline Wozniacki 

Of the final eight in the bottom half, six were ranked outside the Top 100 at some point in 2017, and three - Martic, Mertens, and Rybarikova - weren't even able to play the 2017 edition of the Australian Open. 

Lowest ranking in 2017:

Petra Martic: No.662 (April 9th, 2017)
Elise Mertens: No.127 (January 8th, 2017)
Denisa Allertova: No.131 (November, 26th, 2017)
Anett Kontaveit: No.125 (March 5, 2017)
Magdalena Rybarikova: No.453 (March 19, 2017)

The two most stunning turnarounds have come from 26-year-old Martic and 29-year-old Rybarikova. Both are into the second week of the Australian Open for the first time in their careers, but since coming back from debilitating injuries, both have proven to be second-week Slam stalwarts. 

No.81 Petra Martic has made the second week a three of the last four Slams.

Martic, who was forced off the tour for 10 months due to a back injury, has made the Round of 16 at three of the last four Slams. Her resurgence came at Roland Garros, where she came in as a qualifier and stunned Madison Keys, Anastasia Sevastova, and came within a handful of points of ousting Elina Svitolina. She backed that run up at Wimbledon a month later, again as a qualifier, beating Daria Gavrilova before losing to Rybarikova. 

"I still, at times, look back and I can't believe what season I had," Martic said after defeating Luksika Kumkhum on Friday. "Again, another Grand Slam. I'm just living my dream.

THROWBACK: Petra Martic relishes resurgence at Roland Garros

"When I started, I really had no idea what to expect. I didn't know how my body was going to react, whether I can actually handle so much work. But it's been going great. I'm just really so happy about it."

Magdalena Rybarikova is back where she belongs.

This time last year, Rybarikova was sulking in Slovakia. After enduring surgery on her knee and wrist, Rybarkova's rehabilitation kept her home during the Australian summer. She was left to soak in the sun from her television as she waited to get back on tour.

"I was very unhappy because the goal was to play the Australian Open and I got in with my protected ranking but I was not able to compete because my knee was not feeling well," Rybarikova said after her three-set win over Kateryna Bondarenko. "It took two months more than it was supposed to be."

Her patience paid off. Rybarikova put together a stunning grass-court season to make her first Slam semifinal at Wimbledon, where she ousted Karolina Pliskova and CoCo Vandeweghe, and continued her form through the end of the season to finish in the Top 20. 

"It was an amazing year and I would never have expected that after such a long break. I was just relaxed and just enjoying tennis and just somehow forgot about the past. If somebody told me next year you're going to be in the Top 20 and be in the fourth round of the Australian Open, I would not believe them. It's good to believe."

READ: Rybarikova comeback gains steam at Wimbledon

"So many people were telling me that I was supposed to be Top 20 for so many years. They couldn't understand why I'm not there. It's not easy always. You have to mentally strong, you have to be fit. Maybe I wasn't taking that much care of my body and maybe that was my mistake. Now I take much more care of my body and try to be professional in that. 

"And maybe I'm more mature now. I have more experience. Some people need more time. Some people are good when they're 15 or 20. We're all different." 

Elise Mertens is into her first Round of 16 at a Slam in her Australian Open debut.

Mertens' 12-month story is something for the karmic storybooks. Last January, Mertens was scheduled to play qualifying at the Australian Open but she entered the Hobart International to get some additional matches in before heading to Melbourne. Taking the court for her second-round match against Sachia Vickery, Mertens had every intention of retiring from the match so she could get to Melbourne in time for qualifying. But Vickery retired first, leaving Mertens in a tough position.

The Belgian chose to play through Hobart and skip Australian Open qualifying. She went on to win the title, the first of her career, which vaulted her into the Top 100. Mertens credits the Hobart title for her steep climb up the rankings. 

"That was the key, to be able to see yourself as a top 100 player," Mertens said after defeating Alize Cornet in the third round. "Just believing."  

Asked what she would have thought if someone told her then that she would make the second week of the Australian Open a year later in her tournament debut, Mertens laughed. "I would laugh a little bit. But you have to believe in yourself that you can do whatever you want. If you don't believe in yourself it's going o be hard anyway. I felt it was coming last year, so I think I deserve to be in the fourth round.

Mertens returned to Hobart this year and successfully defended her title and is now riding a seven-match win-streak. The 22-year-old who came up through Kim Clijsters tennis academy in Bree, says she's not ready to pat herself on the back quite yet. 

"I don't really like to think about it yet because it's not finished," Mertens said. "I hope to do more, to do well, and we'll see where it ends. I don't want to say 'Whoo hoo! I'm in the fourth round.' I am, but at this stage, I try to keep myself motivated. There is still energy left and I feel like I can do more."

Madison Keys needed just 41 minutes to move into the third round.

Locker room seed talk.

Madison Keys was asked if she's noticed any different trends so far at the start of the season. She singled out the perceived stricter enforcement of rules by the umpires - which she's in favor of - and the discussion surrounding the announcement that the Slams will move to 16 seeds next year, as opposed to 32 seeds.

"I definitely think the umpires are very strict, which I think is fine," Keys said after her 41-minute win over Ekaterina Alexandrova in the second round. "I do think, especially here because we didn't have it in the weeks leading up, the time to get on court and everything, I feel so stressed all of a sudden. Little things like that. 

"Every time some upset happens, everyone is like, 'We're only going to have 16 seeds. What?' That's pretty much a comment every time something happens. It was something that I think not all of us knew was even happening. 

"Some of us heard about it but it wasn't really a discussion. We were never asked our feelings on it. It just came about. So now everyone is now hearing it and as the tournament is unfolding everyone's like 'Yeah. We need more drama. That's what we need,'" she said with a laugh.

Simona Halep celebrates after defeating Eugenie Bouchard in the second round.

Simona's reason for smiling.

Q. Do you have any story about where that fist pump-smile celebration came from?
SIMONA HALEP: From nowhere. Nowhere.
Second match in China I did it once. Darren said he liked it, so I just kept doing it.

After the first round here, I didn't do it. I couldn't. I was too scared.

Q. So it's going to be a thing?
SIMONA HALEP: Maybe, if I will not forget, yeah (smiling).

There was concern over the status of the World No.1's left ankle ahead of her second-round match against Eugenie Bouchard, but she calmed many frazzled nerves with her dominant 6-2, 6-2 win. Halep moved tentatively at times, but she grew with confidence as the match wore on. The quick win gave her more time to let her ankle heal and should put her in good stead against Lauren Davis on Saturday.

Croatian-American Bernarda Pera celebrates her first Top 10 win over Johanna Konta.

SAP Stat of the Day: Day 4

American Bernarda Pera, who advanced to the third round with a win over Johanna Konta on Thursday, is the first lucky loser to reach the third round of the Australian Open since 1997 (Sandra Kleinova).

SAP Stat of the Day: Day 5

Anett Kontaveit's three-set win over No.7 Jelena Ostapenko was the third Top 10 win of her career. All three wins have come in the last 10 months. 

READ: Get to know Anett Kontaveit

Photo of the Day: Day 4

Caroline Garcia slices a backhand during her second round win.

Photo of the Day: Day 5

Luksika Kumkhum returns a ball during her third-round match against Petra Martic.

Quote of the Day: Day 4

Take it away, Hsieh Su-Wei...

"I think it was too hot today and every time the bug come into the court they're dead. Most of the time I'm picking out the dead body from the bug. It was too hot on court today (smiling)."

Elina Svitolina and Marta Kostyuk, the present and future of Ukrainian tennis.

Quote of the Day: Day 5

We'll give you the last word, Marta...

Q. What did you take from that experience today?
MARTA KOSTYUK: Well, a lot. How much you have to pay Svitolina to have 
one-hour lesson, so I got it for free.

Final Thoughts: Standing ovation goes to Luksika Kumkhum and Petra Martic, who played one of the most entertaining and compelling matches of the first week.... It's great to see a rejuvenated Carla Suarez Navarro enjoying her tennis again.... Jelena Ostapenko picked up a thigh injury in Sydney and said it flared up during her match against Anett Kontaveit.... Ostapenko will play St. Petersburg before heading to Fed Cup.... Magdalena Rybarikova said she only started playing points in Sydney and was worried she was undercooked for Melbourne.... Huge credit to Ashleigh Barty, who has managed a tricky set of opening matches against Aryna Sabalenka and Camila Giorgi, with Naomi Osaka waiting on Saturday.... Madison Keys started off her second-round match by firing four consecutive aces, which she said was a first.