Serena Williams roared into the quarter-finals of the Yarra Valley Classic with a 6-1 6-4 victory over Tsvetana Pironkova on a star-studded day of action in Australia.

The 23-time grand slam champion was in fine form at Melbourne Park, where the biggest names in women's tennis were competing across multiple tournaments, as she sealed a straight-sets triumph to continue her preparations for a shot at an eighth Australian Open title.

In doing so the 39-year-old extended her head-to-head record over the Bulgarian to 6-0 and was delighted to get the job done.

"It's definitely nice to get another win," the American said during her on-court interview.

"She's clearly a great player, so it wasn't easy, but it was good to come through."

Speaking at her media conference, Williams added: "Last time it was an incredible three-set match, so today I was like, 'All right, let's really try and focus and learn to do better than last time.'"

World number one Ash Barty had a less straightforward path to the last eight, requiring three sets to see off Marie Bouzkova.

The home hope ultimately prevailed 6-0 4-6 6-3, with second seed and reigning Australian Open champion Sofia Kenin also taken the distance in a 5-7 7-5 6-2 win over Jessica Pegula, as Nadia Podoroska dug deep to down Petra Kvitova 5-7 6-1 7-6 (9-7).

With Barty now set to face Shelby Rogers, Danielle Collins awaits Williams having impressively defeated third seed Karolina Pliskova 7-6 (7-5) 7-6 (7-3), while Garbine Muguruza overcame Anastasia Pavlyuchenkova 6-1 6-2 to set up a meeting with Kenin.

In the Gippsland Trophy, the top three seeds all dodged upsets, with two-time grand slam winner Simona Halep, 2019 Australian Open victor Naomi Osaka and 2018 Tour Finals champion Elina Svitolina safely navigating through the last 16.

Fifth-seeded Briton Johanna Konta failed to convert two match points as she succumbed to a 4-6 7-6 (12-10) 7-6 (7-4) defeat to Irina-Camelia Begu.

Meanwhile, at the Grampians Trophy there were wins for Anett Kontaveit, Jennifer Brady and Angelique Kerber.

Serena Williams and Simona Halep started the season with straight-sets victories in Melbourne a week before the Australian Open gets under way.

Williams and Halep played in an exhibition event with a crowd of 4,000 watching on in Adelaide last Friday and they were back in competitive action three days later.

Legendary American Williams beat Daria Gavrilova 6-1 6-4 to move into the third round of the Yarra Valley Classic.

The fifth seed struck 27 winners to 15 unforced errors on Margaret Court Arena as she set up a meeting with Tsvetana Pironkova, who ousted Donna Vekic 1-6 6-4 6-2.

Williams said: "It was a good match for me. It wasn't easy at all. It was lots of rallies and lots of movement, and she's from here, so she obviously always plays hard. So it was really good and it felt good to clinch that in the end."

Third seed Karolina Pliskova, Petra Martic, Danielle Collins and Marketa Vondrousova also advanced to the last 16 on Monday.

Elsewhere, top seed Halep is through to the third round of the Gippsland Trophy following a 6-4 6-4 win over Anastasia Potapova.

Halep hit 23 winners and broke twice in each set in what was her first official match since October.

Elina Svitolina, the third seed, beat Andrea Petkovic 6-1 6-4, while Coco Gauff, Ekaterina Alexandrova and Jelena Ostapenko were among the other winners seven days prior to the start of the first grand slam of 2021.

Karolina Pliskova has crashed out of the Abu Dhabi Women's Tennis Open in the second round, while Coco Gauff was also eliminated.

Third seed Pliskova suffered a stunning 6-2 6-4 defeat to Anastasia Gasanova, a qualifier ranked at 292 in the world.

Pliskova fell to defeat in only 75 minutes on Saturday, the first time she had lost to an opponent outside of the top 50 in over a year.

Gasanova, 21, beat Mona Barthel in her WTA main draw debut this week and went on to save five of the six break points she faced to record an even bigger upset against Pliskova.

"It was not me on the court," a stunned Gasanova said after winning in her first match against a top-100 player, booking a last-16 clash against Sara Sorribes Tormo.

"I was so sure of myself. I'm really happy right now.

"I'm really surprised, but when I woke up today, I thought, 'Why not? Why can't I win this match?'

"Actually, I thought I would lose in qualies or something like that, but I just kept pushing myself, like, I need to win and I want to win.

"I had a [2021] goal to be top 200, actually! I don't know if I did this today, but that's my goal. 

"To play against a top-10 player is a great opportunity for me, and it's a great chance to show how I could play."

Pliskova, therefore, starts her year on a negative note, but there were no such problems for second seed Elina Svitolina, who was a 6-4 6-1 winner against Vera Zvonareva.

Last year's Australian Open finalist Garbine Muguruza cruised through with a 6-1 6-4 victory over Aliaksandra Sasnovich and sixth seed Elena Rybakina won in straight sets against Xiyu Wang.

Muguruza will face Maria Sakkari in an intriguing last-16 match next after the Greek ended the hopes of American Gauff with a comfortable 7-5 6-2 victory.

Top seed Sofia Kenin, who made two grand slam finals last year and won in Melbourne, will resume her campaign against Yulia Putintseva on Sunday.