MELBOURNE, Australia — New day, very different opponent: That remains the cruel beauty of tennis.

In the third round of the Australian Open, Amanda Anisimova, a 20-year-old American, got the chance to trade baseline bolts and full cuts with Naomi Osaka, bending low and swinging away to earn an upset.

In the fourth round on Sunday night, the unseeded Anisimova found herself in a more subtle form of combat. If playing the 13th-seeded Osaka was toe to toe, playing the No. 1 Ashleigh Barty was cat and mouse.

Barty, who had to rally to beat Anisimova at the 2019 French Open, knew firsthand the danger posed by Anisimova’s easy power and aggressive mentality and returns. Barty did a fine job on Sunday of giving her the tennis equivalent of bad pitches to hit.

Barty deployed her skidding backhand slice and precise, hard-to-read serve. She changed pace and shape with her topspin forehand, moving Anisimova, who is much more at ease from stable ball-striking positions than on the stretch.

New day, very different result: Anisimova lost to Barty in straight sets, 6-4, 6-3, at Rod Laver Arena.

“Each and every player, there are some similarities, but certainly tactics and the way that I want to play is unique to each and every player,” Barty said. “I try to adapt my game as best as I can.”

Variety is not a panacea. A power player with an irresistible serve can prove too much to handle, which helps explain how Serena Williams has won 23 Grand Slam singles titles. But Barty-style variety is a weapon, too, and she is not the only shape shifter thriving in Melbourne this year.

Barbora Krejcikova, up to No. 4 in the world after her breakout singles season in 2021, is still on a roll. She trounced Victoria Azarenka, a two-time Australian Open champion, 6-2, 6-2, in the afternoon heat on Sunday.

“I take pride in having not just a Plan A but a Plan A, B, C and D,” Krejcikova said in an interview.

Krejcikova will face someone more inclined to Plan A in the quarterfinals on Tuesday: the huge-hitting American Madison Keys, a longtime top-10 player who arrived in Melbourne unseeded after struggling in 2021. But Keys, 26, has been exuding positive energy in the Australian sunshine as she tries to resolve her career-long conundrum: how to remain in command of her emotions in the matches that matter most.

“My biggest mind-set change is just trying to enjoy tennis, take some of that just internal pressure that I was putting on myself,” Keys said on Sunday. “It was honestly freezing me. I felt like I couldn’t play at all. Just taking that away and putting tennis into perspective: that it’s a sport, something that when I was little I enjoyed doing and loved doing it. I was letting it become this dark cloud over me. Just trying to push all of that away and leave that behind last year and start fresh this year.”

So far she is 10-1 in 2022, winning a title in Adelaide before arriving at Melbourne Park, where she has beaten a series of quality opponents including the 2020 Australian Open champion Sofia Kenin; Wang Qiang of China; and Paula Badosa, a new arrival in the top 10 from Spain whose hard-running athletic style was no match for Keys in the fourth round.

But staying calm will become tougher for Keys as the trophy gets closer. For now, she has reached one Grand Slam event final, losing to her close friend Sloane Stephens in a one-sided match at the 2017 U.S. Open in which Keys seemed to freeze.

“I think it obviously gets harder just because you get tighter, and it’s bigger moments,” Keys said. “Even in the finals in Adelaide, I started incredibly nervous, and I felt that. Just acknowledging it, accepting it — not trying to fight it and pretend that it’s not happening — has been probably the best thing that I’ve done.”

Barty will have to clear her own mental hurdles if she continues to advance. No woman left in the draw has won an Australian Open singles title, and the only men’s champion remaining is Rafael Nadal, who faces a tough quarterfinal with Denis Shapovalov, the flashy, left-handed Canadian who has beaten him once and who upset one of the tournament favorites, the third-seeded Alexander Zverev, 6-3, 7-6 (5), 6-3, on Sunday.

Barty is trying to become the first Australian to win the Australian Open singles title in 44 years. The last was Christine O’Neil, who prevailed in 1978 over a relatively weak international field. But Barty, entrenched at No. 1, is up against much stronger opposition. Unlike O’Neil, who was unseeded, Barty is the focal point whenever she plays in Australia, even if she was spared from pretournament scrutiny this year because of the furor over Novak Djokovic’s vaccination and visa status.

But Australians are paying closer attention now, wearing their “Barty Party” T-shirts; shouting, “C’mon Ash”; and watching in large numbers on television.

Rod Laver, the former Australian great who made the trip from his home in California, has said that Barty is ready to do “something special.” He was in the stands at Rod Laver Arena on Sunday night.

“It’s so nice to have him enjoying his own house, enjoying his own court,” Barty said. “He was unbeatable. I’m certainly not.”

Barty, who skipped the end of last season to return to Australia to recharge, has yet to advance past the semifinals in singles at the Australian Open. To get that far again, she must defeat Jessica Pegula, an American who is more consistent and poker faced than Anisimova; her game is more difficult to read, too. More counterpuncher than puncher, she got the balance between patience and aggression just right in her straight sets victory in the fourth round over Maria Sakkari, who beat Pegula in a three-set thriller in Miami last year.

“I feel like Ash is so tactical in everything she does,” Pegula said. “Really a smart, like perfect, kind of tennis player in that way.”

But the 21st-seeded Pegula, not the quickest or most imposing athlete on tour, has made her leap into the elite by widening her range even if the core of her game remains her pure, relatively flat groundstrokes.

“She’s able to hold the baseline really well,” Barty said. “Her swings are quite linear, and she gets a racket behind the ball and swings through the path. The ball comes at you at a different trajectory, and her ability to absorb pace and then add to it when she wants to is exceptional. It’s going to be a challenge for me to try to push her off that baseline and make her uncomfortable and feel like she has to create. But I know she’s also going to be doing the exact same thing to me, trying to make me uncomfortable.”

That push and pull is the essence of tennis, and what works on a Sunday may no longer work come Tuesday.