Two all-seeded three-setters kicked off the Miami Open fourth round as No.1 Ashleigh Barty defeated No.14 Victoria Azarenka 6-1, 1-6, 6-2 and No.5 Elina Svitolina came back to beat No.9 Petra Kvitova 2-6, 7-5, 7-5.

All four players on court have been ranked in the Top 3, and lived up to their billing in delivering unpredictable momentum shifts, fluctuating scorelines and matches that were too close to call until almost the end.

Defending champion Barty extended her Miami winning streak to nine and her head-to-head against Azarenka to 2-1 by coming out on top of a tactical battle of wits. Azarenka's intention of using her own formidable backhand to attack Barty's was clear from the outset, but the Australian proved able to withstand the barrage. Both Barty's slice and less-used drive were solid in a dominant opening set which, appropriately, she sealed with a blazing backhand pass.

Vocally exhorting herself, Azarenka raised her game to wrest momentum back. This year, Barty has showed a pattern of fast starts followed by second-set dips, and here she followed up her 10 winners in the first set with just two in the next - and 11 unforced errors. Azarenka pressed home her advantage with a series of scorching backhands and returns, winning every point on the Barty second serve.

"Playing against Vika you have to be ready right from the first point," Barty said. "I felt like I did that well in the first set and she gave me a few cheap errors. Then through the second she lifted her intensity. I probably wasn't sharp enough in those first couple of games to be able to go with her, particularly with new balls. When Vika is ahead and she's a front runner, she's the very best in the world."

This set the scene for a battle royale in the decider, and the middle of the set provided just that. The heart of the set was a pair of titanic holds in which both players threw their highest quality at each other. Both saved two break points and survived four deuces, but it was Barty's hold for 3-2 that won the World No.1 the scoreboard momentum.

Barty had won that game as an Azarenka forehand had sailed long - the start of an unfortunate descent into error for the US Open runner-up. Azarenka had committed 17 unforced errors across the first two sets, but tallied 18 in the third. This included five in the final game alone, frequently from a position of control in the point. A netted Azarenka volley sealed a quarterfinal berth for Barty on the top seed's third match point.

"The first two sets probably had some pretty big momentum swings, but I think the third set was a real tussle," assessed Barty afterwards. "Particularly through that middle part, those games in the third set turned out to be pivotal, but it was just about staying the course and over time trying to execute my game plan as best I can, and try and bring it back onto my racquet where I felt I was in control a little bit more in that third set and had a little bit better court position."

"It's about not relenting, not giving up, regardless of whether you lose a set or you're down a break."

- Ashleigh Barty on winning 17 of her past three-set matches.

Having also saved a match point against qualifier Kristina Kucova in the second round, this result takes Barty's record to 17 wins in her past 20 three-set matches.

"I don't like rollercoasters, so I think rollercoasters during a tennis match is enough for me," she joked.

Barty's approach to those is typically level-headed.

"Of course there is [frustration], but frustration is not always a source that's going to deter me from trying to go about things the right way," she said. "It's just about staying in the fight. It's about not relenting, not giving up, regardless of whether you lose a set or you're down a break, whatever it might be. I think it's just about trying to hang around. I have worked my backside off, off the courts to make sure that I feel like I'm in a good physical condition to play tennis and I'm ready to play matches, but it's just about competing. Competing over a long period is something that I find really fun and really enjoyable."

Like Barty, Svitolina seemed at her opponent's mercy for the duration of a set. Kvitova got off to a lightning start, landing 70% of her first serves and striking 16 winners to just seven unforced errors in the opening set. The Czech went all out on return, relentlessly punishing Svitolina's paltry 33% first serve percentage.

But for all the scorching returns that left Svitolina flailing, Kvitova's tactics were high-risk. The second set saw the Ukrainian improve slightly, landing 52% of her first serves, but Kvitova's full-on aggression now resulted in a cascade of missed returns - and a sequence of straightforward holds for her opponent.

Building confidence as she gradually worked her way into the contest, it was Svitolina who created the only break points of the set at 5-5. A wide backhand from Kvitova conceded the break, and Svitolina forced the decider with minimum fuss.

The 2018 WTA Finals champion seemed to be in the driver's seat as she took a 3-0 double-break lead. After losing a four-deuce tussle to drop serve in the first game of the final set, Kvitova sprayed unforced errors with alarming frequency, and bent double between several points.

The two-time Wimbledon champion wasn't ready to go quietly, though, and found the energy for another fusillade of winners as she retrieved both breaks. Kvitova would finish the match with an impressive ratio of 42 winners to 27 unforced errors. But as in the second set, it was Svitolina who was more solid at its tail end.

A gorgeously angled backhand return garnered Svitolina another break for 5-4. Though she failed to serve out the match at the first time of asking, she continued to pressure Kvitova, who coughed up her eighth and ninth double faults to drop serve again. This time, Svitolina made no mistake. Having played too passively serving at 5-4, she stepped up with more offensive ball-striking to take victory.

"I think physical aspect was one of the main points," said Svitolina about the key to the win. "I felt not bad out there. For sure it was not easy conditions, but I was trying to fight and was trying to find a way of one extra ball over the net, to try to be aggressive as well. So I think physical aspect really played a big role."

2021 Miami Highlights: Svitolina comes from behind to outlast Kvitova

Though Svitolina had only beaten Kvitova twice in nine previous encounters, the first of those was a key milestone: her maiden Top 10 win in the second round of Cincinnati 2014. The quality of Kvitova's game has given Svitolina an appreciation of their rivalry.

"It's always a big battle to play against her because she strikes the ball very clean and you have to adjust extremely quick," she said. "Today in the first set, OK, I didn't play my best but, you know, she was stepping in. She was striking the ball extremely good. So for me, always a big challenge to play against her but also gives me lots of positives from winning those kind of matches. She's a top player. She has great results each year. She always brings the best out of you. That's why I really enjoy those kind of battles."

It is the second time this tournament that Svitolina has snatched victory from a set down, and puts Svitolina into her second Miami quarterfinal following a run to this stage in 2018. A semifinal debut will be at stake when she takes on either Anastasija Sevastova or wildcard Ana Konjuh next.

The third clash of seeds on Day 7 was also won by the higher-ranked player as No.7 Aryna Sabalenka trounced No.19 Marketa Vondrousova 6-1, 6-2 in just 67 minutes. The Belarusian had never gone beyond the second round of Miami before this year, and came into this clash having lost two of her three pro meetings with Vondrousova. But 28 winners to the Czech's seven underlined the scale of her dominance as she set up a last-eight tilt against Barty.