Two-time Australian Open champion Naomi Osaka is in doubt for the Melbourne major after her quest for a first title in four years ended in heartbreak in Auckland.

The former world No.1 retired with an abdominal strain after winning the opening set of the Auckland Classic final against Denmark's Clara Tauson.

Osaka, who won the last of her four grand slam trophies at Melbourne Park in 2021, returned to the circuit a year ago after a long maternity break but struggled for consistency and ended the season ranked outside the top 50.

She showed glimpses of just how devastating she can be in Auckland on Sunday and grabbed a double break against Tauson with powerful hitting on both flanks in the opening set, which she wrapped up 6-4, despite dropping serve late on.

But the 27-year-old from Japan had tears in her eyes as she received treatment on court before the start of the next set and then threw in the towel, sparking concern about her fitness for this month's Australian Open starting on January 12.

It was the second injury-related retirement for Osaka in as many tournaments. She suffered a back injury at the China Open in October, retiring against Coco Gauff in the round of 16, and that problem put a premature end to her 2024 comeback.

"I just want to thank everyone for welcoming me to such a beautiful city," Osaka said on Sunday, having last played in Auckland in 2017.

"I had a lot of fun playing here, and I'm really sorry about how it ended. I hope you enjoyed the tennis that we did play, and I'm just really grateful to be here."

Osaka was playing in her first final since 2022, while Tauson had waited even longer, contesting her first tour-level final since 2021.

Tauson is the first player from Denmark to win this title. Her fellow Dane, former World No.1 Caroline Wozniacki, was a two-time runner-up in 2015 and 2018.

"It's not the way you want to win any match, and especially not the final," Tauson said.

"I felt like I had some more tennis to show today.

"I'm looking forward to going to Australia," she said. "Hopefully I can back (this) up with some great results there.

"It's definitely a nice start to the season with the title, it's my first one since 2021, so it's been a while for me."

With AAP