Former finalist Petra Kvitova suffered a "painful" first-round defeat by Sorena Cirstea on the second day of the Australian Open in Melbourne.

Kvitova, who lost to Cirstea in the second round at last year's tournament, has not gone beyond the third round of a Grand Slam since the 2020 US Open.

US Open runner-up Leylah Fernandez also lost on Tuesday, while second seed Aryna Sabalenka fought back to advance.

Kvitova, often a crowd favourite in Melbourne, made 39 unforced errors and hit six double faults against Cirstea.

"This loss is very painful - in the first round after like a whole month [in Australia]," Kvitova said.

"I did have some health issues the whole month as well, so it was tough to practice.

"The season is long, so I hope that it will turn around."

Canadian teenager Fernandez lost 6-2 6-4 to Australian wildcard Maddison Inglis, who was tearful after winning her first match at Melbourne Park.

"I am super stoked, I am shaking," the 24-year-old said.

"I watched a lot of Leylah at the US Open. We were there supporting, watching her singles matches, then I would go and watch her doubles matches. She's such a lovely girl."

Belarusian Sabalenka fought back from a set down to beat Storm Sanders, the Australian world number 128, 5-7 6-3 6-2 and register her first win of the season.

Simona Halep, runner-up in 2018, continued her good form with a 6-4 6-3 win over Poland's Magdalena Frech, while third seed Garbine Muguruza also advanced after beating rising French star Clara Burel 6-3 6-4.

However, 2016 champion Angelique Kerber fell in the first round for the second year in a row as she was beaten 6-4 6-3 by Estonia's Kaia Kanepi.

Australian veteran Sam Stosur, playing in her final singles event, recovered from a set behind to overcome American Robin Anderson 6-7 (5-7) 6-3 6-3.

Stosur will continue playing doubles until the end of the 2022 season, but said she would retire from singles after being given a wildcard into the Australian Open.

She will face 10th seed Anastasia Pavlyuchenkova next after the Russian beat Anna Bondar of Hungary 6-2 6-1.