Naomi Osaka leaves the press conference after her withdrawal from the WTA Finals tournament in Shenzhen, Guangdong Province on Tuesday. Photo: IC

 
Japanese sensation Naomi Osaka vowed to be fit and firing for her Australian Open title defense after her season was abruptly cut short by injury at the WTA Finals on Tuesday.

The two-time Grand Slam winner, who was on a hot streak of 11 matches, withdrew from the season finale with a shoulder problem - the second straight year she has been forced out due to injury.

Third-ranked Osaka, who retired during last year's edition of the lucrative year-ender with a hamstring injury, had been due to play world No.1 Ashleigh Barty later on Tuesday.

Her place in Red Group will be taken by world No.10 Kiki Bertens, who will now face Australia's Barty in Shenzhen.

The 23-year-old Osaka said she was unsure of the severity of the injury to her right shoulder, which she initially hurt during her triumph in Beijing earlier this month.

"I felt it immediately [in her first match on Sunday]," she told reporters. "When I woke up the day after it was throbbing.

"It's just a little bit painful. I don't think it's something that I would immediately need surgery for."

Osaka had started the $14 million round-robin tournament with a tough three-set victory over Petra Kvitova on Sunday to extend her winning streak, after titles in Beijing and Osaka.

She had been determined to make amends for a disappointing WTA Finals debut last year, when her winless campaign ended with a tearful retirement against Bertens.

"I thought I was playing well. I definitely wanted to win here," Osaka said.

Once she recovers from the injury, Osaka's attention will turn toward the Australian Open in January, where she will defend a Grand Slam title for only the second occasion.

"I just want to train really hard," Osaka said. "Last year during the off-season, I trained really hard for Australia. I felt like going into the Slam I was really fit. I just want to try to duplicate that."

Even though it ended prematurely, Osaka was pleased with her back end to the season after a tough stretch where she suffered a stunning first-round exit at Wimbledon followed by a lacklustre US Open title defense.

She rated 2019, which started on a high in Melbourne, as a more pleasing season than last year, when she made her Grand Slam breakthrough by beating Serena Williams in a tempestuous final at Flushing Meadows.

"[Losing at Wimbledon] was the lowest emotional feeling I've ever felt," she said. "After the US Open, I set goals. I was able to win tournaments. That was definitely the highlight... I definitely think that this year was somehow better [than last year]."