MELBOURNE, Australia - No.18 seed Garbiñe Muguruza overcame a stern test from the dangerous floater Timea Bacsinszky to move into the second week at the Australian Open for the fourth time in her career, emerging 7-6(5), 6-2 after one hour and 54 minutes from their third-round tilt.

The Spaniard had made her debut in the fourth round of a major here in 2014, repeated that result in 2015 and reached the quarterfinals in 2017; Flinders Park is her second most successful major in terms of consistency over the years following Roland Garros, where Muguruza has featured in the second week for the past five years. Indeed, 2015 saw her defeat Bacsinszky 6-3, 4-6, 6-0 in the third round - the first of four straight wins over the Swiss player, a streak only halted by Bacsinszky in their most recent meeting in the first round of Madrid in 2017.

"She has a very unique style of game, very smart, very talented," said Muguruza afterwards. "We played I think four or five times already. All of the matches were very difficult. I know I was going to have to fight a lot to get it. So it counts a lot, this one."

Two nights ago, Muguruza had come through a marathon in the small hours, with her 6-4, 6-7(3), 7-5 win over Johanna Konta beginning at 12.30am - officially the latest ever start in the tournament's history - and ending at 3.12am. But the match had been of superb quality, and despite the fatiguing time and length, the two-time Slam champion managed to carry much of that form over today.

Nonetheless, the self-described "morning person" discussed how tough it had been on her body clock and usual plans for an off day. "I think I went back to sleep around 5, so it's very messed up," she explained. 

"You don't know what to do the next day because you wake up very late. You're still tired from the match. It messed up a little bit the normal circumstances of a day off."

Having woken at 11am, Muguruza opted out of practising yesterday. " I will not forget to play tennis in a few hours," she said. "I didn't play. I woke up late. I prioritized. Rest a little bit, work with my physio, just get a little bit of distance, recover for today because I knew was going to be a battle."

Bacsinszky certainly didn't make life easy for her. The 29-year-old may be ranked just No.145 - but it has taken her just five months to jump 600 spots from a depth of No.745 in August, a mark of how the former World No.9's comeback from hand surgery has clicked recently. Just last week, Bacsinszky upset Anastasija Sevastova en route to the Sydney quarterfinals.

Read more: 2019 Scouting Report: Bacsinszky poised to bounce back

And in a tightly contested first set saw the momentum swing back and forth between the two players, it was Bacsinszky who twice went up a break - with a delectably angled reflex half-volley in the very first game, and after converting her fourth break point in the fifth game. But both times, Muguruza responded aggressively, punishing any serves and short balls in her strike zone with blitzed backhands. The 25-year-old also had a successful answer to the beguiling mixture of spins and paces that Bacsinszky threw at her: moving relentlessly forward, Muguruza was lethal and precise on the overhead, nailing almost every one today.

Indeed, the Muguruza backhand would be today's key shot, both her most dangerous weapon but also prone to loose mistakes. It provided 10 winners to 15 unforced errors in the first set - but the former World No.1 was focused and lethal on big points. A marvelous lob brought up set point on the Bacsinszky serve at 6-5 - and although the two-time Roland Garros semifinalist saved it with a sharp off backhand winner of her own, Muguruza would quickly snatch a 5-2 lead in the tiebreak that she ultimately closed out with an unreturnable serve.

An early exchange of breaks featuring a number of exquisite extended rallies initially promised a second act that would also go down to the wire, but Muguruza's serve and backhand were now fully dialled in. The six-time WTA titlist would break for 3-2 with more booming returns, survived a three-deuce tussle to hold for 4-2 - and then was in the clear.

Eight of the final 11 points went to Muguruza - and although Bacsinszky battled to the end, saving a second match point with a gloriously angled wrong-footing backhand winner, the former Roland Garros and Wimbledon champion closed the deal on her third with - naturally - another ferocious backhand that flew past her opponent.

Muguruza's first week has been an arduous test - and it doesn't get any easier from here, with No.7 seed Karolina Pliskova awaiting in the fourth round.