The men concluded the first day of fourth-round play with two wildly entertaining matches. Once the second day got underway, the women answered with two of their own.

First, Dominika Cibulkova got a step closer toward returning to the final with a 6-2, 3-6, 6-3 win over the unseeded but improving two-time champion, Victoria Azarenka. Serena Williams, a five-time champion in Melbourne, then outlasted the dangerous Garbine Muguruza—who had beaten the world No. 1 at last year’s French Open—2-6, 6-3, 6-2.

The scores do neither of these matches justice; it’s hard to believe that none of the six entertaining sets lasted more than nine games. But the totals were greater than the sums of the parts. Here are three thoughts on these two terrific three-setters:

1. Domi’s Nation. Cibulkova rode her runner-up run at last year’s Australian Open to immediate success outside the island continent. She won a title in Acapulco a month after Oz, then reached the quarterfinals of Indian Wells and the semifinals of Miami. Once the tour switched to clay, however, Cibulkova got stuck. She went 10-17 in her next 27 matches, the low point occurring at the next hard-court major, the U.S. Open. At Flushing Meadows, the 12th-seeded Slovak lost her opening match to junior CiCi Bellis, who at the time was ranked outside the Top 1,000.

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Three Thoughts: Serena, Cibulkova top improving opponents in three-setters

Three Thoughts: Serena, Cibulkova top improving opponents in three-setters

But Cibulkova is once again finding her best form at The Happy Slam, hitting the reset button at an opportune moment, considering the sizable amount of rankings points she stands to lose with an early exit. “It's not only about the place,” said Cibulkova about Melbourne, “it’s also about the timing of the tournament. You know, the beginning of the year I always feel fresh. I always feel good from the off-season. I always work really hard. Obviously everybody is working really hard in the off-season, but that's what gives me the confidence. That's how I feel good on the court.”

Against Azarenka, the pint-sized 25-year-old again punched above her weight class. She didn’t fear errors in taking her shots down the line—a sound strategy against Azarenka, who embraces the extended rally. She struck 44 winners overall, including 21 in the first set alone. Capable of brutal brilliance with both her forehand and backhand, Cibulkova also forced Azarenka to go for more than she’s perhaps comfortable with at the moment.

On the whole, Azarenka should be encouraged with her performance Down Under. Largely absent from the tour in 2014, she beat Sloane Stephens and Caroline Wozniacki, and with more hard-court play coming up, it would be surprising to see her unseeded any many more Slams. For Cibulkova, streaky as they come, it will take a strong obstacle to trip her up. Speaking of that…

2. Serena, and her fans, should be concerned. Before this tournament, I said that Serena’s early-round matches would be more telling and indicative of performance than in some time. For years, they were cakewalks, but Williams reached the quarterfinal round of just one major in 2014. We shouldn't take anything Serena does for granted, considering her age and the emotional investment she seems to put into every single point. It can be beautiful to watch Williams at 33, and it can also be painful.

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Three Thoughts: Serena, Cibulkova top improving opponents in three-setters

Three Thoughts: Serena, Cibulkova top improving opponents in three-setters

Serena has posted some lopsided scores—three 6-0 sets through her first three matches. But none of them were simple matches altogether, and she’s been taken to three sets twice in four rounds. Of all things, she’s not cleaning up games as quickly with her serve as she typically does. Serena won only 61 percent of her first-serve points in her third-round win over Elina Svitolina, and against Muguzura won just 42 percent of her second-serve points. That was largely due to the Spaniard’s return, which is exceptional. She sent Serena’s pace right back at her with an aggressive approach and raced to a double-break lead in the first set. Once the match went the distance, Muguruza earned six break points in Williams’ first service game.

But that 22-point game was the turning point of the match, even if Serena didn’t think so in her post-match interview. Even if there were still some lingering double faults, Serena rediscovered her serve in that game, saving some of the break points thanks exclusively to the shot. With six break chances, the actions of both players contributed to the outcome, but Serena’s play was the greater cause of the effect. She broke Muguruza the next game and wasn’t threatened thereafter.

It was a huge win for Williams, avenging her loss to Muguruza on the terre battue last season, but we haven’t seen full-flight Serena yet this Australian Open. She may need it if she’s going to contend with the surging Cibulkova.

3. Garbine Muguruza is going to be great. “She did everything right,” Serena said about her opponent after she’d finally put her away. It’s tough to argue with that, as the 21-year-old demonstrated an assertiveness that will serve her well on any surface and against any player.

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Three Thoughts: Serena, Cibulkova top improving opponents in three-setters

Three Thoughts: Serena, Cibulkova top improving opponents in three-setters

Muguruza’s return, as I mentioned, was exemplary in combatting the game’s greatest server, and her forehand was just as impressive. She cracks the shot in such a way that surprises even those who’ve seen it before, including players. Muguruza has a slightly abbreviated swing on all of her shots, including her serve, but no power appears to be sacrificed for that added control. She won’t win games with her serve alone, but service winners aren’t out of the question. They’re not critical, though, as her all-around game is already superior to those of most of her opponents.

Murgurza’s obvious talent hasn’t gotten her to where she wants to be—yet. The signs were there, though, in both her win over Serena last year and her loss today, that Muguruza can handle the best opponents and on the big stages. Once she improves her consistency and discipline, the Top 10 seems a likely destination.