Tokyo, Japan - Four-time grand slam champion Naomi Osaka admitted she has not had "the best year" but is still "pretty happy with where I am now."

Osaka – a former world number one – has slipped to 48th in the latest WTA rankings, and has not won a match at a grand slam since she beat Madison Brengle in the second round at the Australian Open in January.

The Japanese-born star was eliminated in the first round at the French Open and US Open this year, and did not compete at Wimbledon due to an Achilles injury.

Having taken some time away from tennis last year to focus on her mental health, Osaka has struggled to get back to her previous high levels.

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However, speaking ahead of the Pan Pacific Open in Tokyo, she seemed calm about the situation, as she suggested she is improving both on and off the court.

"It was kind of hard at first, just because I feel like I should be somewhere where I'm obviously currently not," the 24-year-old athlete said.

"I think for me, it's more about being at peace with myself. I know that I'm here for a reason."

Osaka has seven career singles titles to her name, including four majors – she has won twice at Flushing Meadows and twice at the Australian Open.

"This year has not been the best year for me, but I've learned a lot about myself," she added.

"Life is ups and downs, and this year was more down than up, but overall I'm pretty happy with where I am now."

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She is preparing to defend her title at the Pan Pacific Open, which has not been held in three years due to the coronavirus pandemic.

"It feels really weird to hear that I'm the defending champion because it was the last time I played in 2019," she said.

"I would love to win it again, but I think just taking it one match at a time... and also just playing in front of a crowd in Tokyo again, because the Olympics was crowdless, so it will just be nice to see people."

Osaka's first match against Daria Saville on Tuesday ended early as the latter retired with knee pain. Osaka will enter the second round with a match against Haddad Maia.