Serena Williams and Naomi Osaka coasted into the third round of the Australian Open, while Simona Halep was again taken to three sets before triumphing and Garbine Muguruza won a marathon encounter.

Seven-time champion Serena made light work of Eugenie Bouchard on Thursday, winning 6-2 6-2 in just 70 minutes on Rod Laver Arena, while Osaka was even quicker in beating Tamara Zidansek 6-2 6-4.

Halep complained about an injury in the second set of her 6-3 6-7 (5-7) 6-4 triumph against Sofia Kenin and Karolina Pliskova fought from a set down to beat Madison Brengle 4-6 6-1 6-0.

Muguruza and Johanna Konta did not begin their match until 00:30 local time (13:30 GMT) - the latest start in Australian Open history - and played for two hours and 42 minutes until the former finally won in three sets.

Elina Svitolina, Venus Williams, Madison Keys and Elise Mertens also recorded wins on day four.

SPEEDY SERENA

Serena has spent less than two hours on court at Melbourne Park this year, but she does not think winning quickly in the early rounds is important.

"I don't think it's key. I think what's key is just going out there and playing really well, just doing the best that you can do," she said.

Next up for Serena is Dayana Yastremska, who overcame Carla Suarez Navarro 6-3 3-6 6-1 to reach the third round in only her second main draw appearance at a major.

OSAKA UNCOMFORTABLE IN NEW ROLE

Last season provided a breakout year for Osaka, who claimed her first grand slam title at the US Open.

When told Zidansek said she felt a bit star-struck going up against Osaka, she responded: "I feel old!

"Wow, that's kind of crazy. I don't know. It's weird. For me, when I play Venus or something, I'm star-struck, too. It's a bit weird when you tell me there's someone I'm playing that feels that way."

HALEP STRUGGLING THROUGH, MUGURUZA'S MARATHON

Halep's first match – and only one prior to the Australian Open – after returning from a back injury that kept her out from September was a loss to Ashleigh Barty at the Sydney International

Although Kaia Kanepi and Kenin have taken her to three sets at Melbourne Park as she battles with a leg problem she branded "nothing dangerous", she is still confident of competing for the title.

"I can say I had a feeling that even if I didn't have so many matches before the tournament, I have a chance, because the tennis was there, even with Barty. Even if I lost that match, I felt that I played a good level of tennis."

Muguruza, meanwhile, celebrated her 6-4 6-7 (3-7) 7-5 victory at 3.12am local time and the 35 games in the contest were the most in an Australian Open women's singles draw match.

What next for Muguruza? "I go to breakfast," she wittily said. 

SVITOLINA SQUIRMS

Svitolina had Gael Monfils in her box as she cruised past Kuzmova and let out an awkward laugh when asked about his presence on court.

When quizzed further in a news conference, she said: "We are here for tennis, so that's why it was a bit strange."

"I didn't really expect to be asked that because I just finished my match."

"He's there for me, he's supporting me, and I'm there for him as well. He understands, I understand, what we're going through. It's great."