MELBOURNE, Australia -- It's become as routine as her service motion: Serena Williams wins a match, walks to the net, hugs her opponent and then consoles the woman she just defeated. It's happened more than once here in Melbourne, and after her third-round 6-2, 6-1 win against 18-year-old Dayana Yastremska on Saturday afternoon, microphones caught Williams attempting to cheer the young Ukrainian, who was in tears. "You did so well," Williams said. "Don't cry. You're so young."

No scene better demonstrates the disparity between Williams, a 37-year-old, 23-time Grand Slam champ, mother and the most famous woman in tennis, and so many of the players she faces. Williams has sandblasted her way through the first three rounds of this draw, dropping only six games and spending a grand total of 3 hours, 6 minutes on court through the first week of the tournament.

She's expending so little mental energy in her matches that when they end, she has more to give, switching as if by autopilot into the role of mentor and motherly ambassador of the game. Her opponents, on the other hand, are learning it's tough to win a match while in awe of the woman on the other side of the net.

But that ends Monday, when Williams faces Simona Halep in the fourth round of the Australian Open. Because while it's one thing to beat unseeded players to tears, it's another altogether to play the current world No. 1. Williams should know. She's been in Halep's position many times in her career, and she said the challenge of having to play through the top-ranked player in the world is one she welcomes on the way to earning a potential eighth Australian Open title.

"I honestly would love to face the world No. 1," Williams said Saturday, a few hours before Halep defeated Venus Williams in straight sets. "I haven't played the world No. 1 since I've been back. It will be good."

At the beginning of this tournament, few people picked Halep to be a threat deep into the draw. She arrived in Australia only days before the tournament, hadn't won a match since August and spent an extended offseason at home in Romania allowing her body to heal from a nagging back injury. But after gutting out two marathon three-setters in the first two rounds -- and a commanding win over Venus on Saturday night -- that changed.

Serena Williams has won 20 consecutive sets at the Australian Open. ULIAN SMITH/EPA-EFE/REX/Shutterstock

Against Venus, Halep played her best match in months. She was aggressive and moved incredibly well. She committed only 12 unforced errors and was 5-for-10 on break points. She looked like a woman very much poised to win a second Grand Slam and become only the third player to beat the Williams sisters back-to-back in a major.

"She played pretty flawless today," Venus said. "She played a great match." Players often talk about playing themselves into shape at Slams. This fortnight, Halep has been playing herself into mental shape, becoming more confident with each victory. Since earning the No. 1 ranking for the first time in October 2017, Halep's biggest opponent has often been the voice inside her own head. Saturday night, that voice was strong and confident.

In her postmatch news conference, Halep talked about experiencing a mindset shift during the offseason, when she began to own her place at the top of the world rankings. She will need that confidence more than ever against Serena, a player Halep has beaten only once in nine attempts in her career.

"It's going to be [a] huge challenge for me," Halep said. "It's always great to play against Serena, because every time I have to learn something from the match. I think I have to play similar [to how I played Venus]. I have to be smart, aggressive. I have to move her and give my best. I will go out there and play natural and with confidence."

play 1:02 Serena uses Billie Jean King as inspiration During her post-match interview, Serena Williams attributes success in not only tennis but in women's sports to Billie Jean King, whom Williams calls an inspiration.

With those comments, Halep betrayed perhaps her biggest advantage. While Serena has yet to face a true challenge at this tournament, Halep couldn't have asked for a better pre-Serena tuneup than to play her older sister, the woman Serena credits with honing her game and competitive drive, and with whom she shares a similar game. Although when Halep initially saw the draw, she might not have looked at it that way.

"Every time I face the sisters is a big challenge for me," Halep said. "I just want to try to play my best tennis because I have nothing to lose against them."

Only 10 months into her return from maternity leave, Williams is in a similar position. Despite a reputation that precedes her when she walks onto center court, Williams is currently ranked No. 16 in the world. If she loses to Halep in the second week of a Slam, she could play it off as a noble setback in a long comeback, a stumbling block on the way to rewriting history.

It's a fascinating matchup. Both women have legitimate claims to being the top player in the game. The question is, which woman will play like it come Monday?

"In my opinion, to be No. 1 in the world and to be the best player in the world, it's a little bit different," Halep said. "In this moment, I'm No. 1 in the world, so I will take that.

"But for sure, [Serena's] the best player in the world because she's won so many Grand Slams. I cannot compare my results to her. But in this moment, I am confident and I'm positive. She was [intimidating] when I was younger, but now I don't feel intimidated. I have huge respect for her because she's a great champion, but she's just an opponent next round."

At least that's what she needs to tell herself.