World number two Simona Halep clawed her way into the Australian Open third round after fighting back from the brink against Ajla Tomljanovic.

The Romanian, a finalist in 2018, trailed 5-2 in the deciding set before beating the Australian 4-6 6-4 7-5.

A nervy Tomljanovic failed to serve out the match at 5-3 and Halep showed her experience to secure a remarkable win.

"In the end I think mentally I was a little bit stronger, had the desire to win the match," she said.

In front of a rowdy Margaret Court Arena, Halep cut a frustrated figure, frequently yelling and gesturing at her coach, Darren Cahill.

However, when Tomljanovic faltered at the key moment, the two-time Grand Slam champion forced her way through her opponent's defences and shook her fists in celebration at the end.

"I am really happy I can smile now," Halep said in her on-court interview.

"I was not that positive when I was talking to myself, I was just blaming myself that I am not strong enough to play against her."

Halep will play Russia's Veronika Kudermetova next.

Serena Williams earlier breezed through after Petra Kvitova and Bianca Andreescu became the biggest casualties on the third day at Melbourne Park.

American Williams, who has dropped just five games in her two matches so far, beat Serbia's Nina Stojanovic 6-3 6-0.

Czech ninth seed Kvitova, who reached the final in 2019, was stunned 6-4 1-6 6-1 by Sorana Cirstea of Romania.

Andreescu, playing in her first major since winning the 2019 US Open, lost 6-3 6-3 to tricky Taiwanese Hsieh Su-wei.

Naomi Osaka underlined why she is one of the favourites for the title with a swift 6-3 6-3 victory over Caroline Garcia of France.

Halep shows trademark grit

Halep, a semi-finalist in 2020, has a winning record against Tomljanovic but all but one of their four matches have gone the distance.

Tomljanovic fought back from an early break down to take the opening set but Halep, one of the best returners on the tour, did not fade away.

She came out on top in a second set that featured five breaks of serve but Tomljanovic raced away with the decider to open up a 5-3 lead.

However, Tomljanovic, serving for the biggest win of her career, grew tight, sending a forehand long and then double-faulting to allow Halep back in.

The Romanian reeled off the final five games, despite looking overwhelmed at times by Tomljanovic's power, and she was warmly received by the crowd after the two hour 34 minute win.

'Relaxed' Williams making serene progress

Williams, who is chasing a record-equalling 24th Grand Slam title, is doing well at conserving her energy, having spent just 125 minutes on court so far.

Since returning from maternity leave in 2018, the 39-year-old has reached four major finals but fallen short each time with the pressure of matching Australian Margaret Court's record seeming to weigh her down.

But there has been no sign of that so far here as she overcame early resistance from world number 99 Stojanovic before cruising through to a third-round meeting with 19-year-old Russian Anastasia Potapova.

"I wasn't thinking as much in the second [set], as I was in the first," said Williams, who hit 27 winners. "I just think when I'm not thinking it's more or less like I'm just totally relaxed.

"Even with my serve, I overthink it. But when I don't think about it... it goes in."

Andreescu's comeback halted and 'unexpected' win for Cirstea

Andreescu and Cirstea were among the 72 players confined to their hotel rooms and unable to train on their arrival in Australia as part of a 'hard' quarantine imposed after there were positive coronavirus tests on their charter flights.

Eighth seed Andreescu - who had been sidelined for 15 months with a knee injury - appeared frustrated and uncomfortable against Hseih, falling to a 4-0 deficit in the first set before mounting a brief comeback.

In 30C temperatures, the Canadian was broken early in the second set and surrendered the match on a double fault.

Cirstea had complained before the tournament about the 14-day quarantine, saying the rules changed "overnight" after a passenger on her flight tested positive for Covid-19, and she said she had surprised herself against Kvitova.

"I think it's impressive to come out of 15 days without hitting a ball and competing the way I did today, I wasn't expecting it," the Romanian said.

Who were the other winners on Wednesday?

Last year's finalist Garbine Muguruza continued her good form on the hottest day of the tournament so far with a 6-3 6-1 win over Russian Liudmila Samsonova.

Belarusian seventh seed Aryna Sabalenka also advanced, beating Daria Kasatkina of Russia 7-6 (7-5) 6-3.

However, Venus Williams lost to Sara Errani 6-1 6-0 - a scoreline that does not do the American justice after she played on with an injury.

Williams twisted her right ankle in the first set and had heavy strapping applied to it, as well as having further treatment to her already taped left knee.

The tearful 40-year-old looked as though she would withdraw but she composed herself and returned, despite being unable to serve or move properly, to see out the match against the Italian.