Tokyo, Japan - Former world number one Naomi Osaka said she would "keep attempting" to get back to her Grand Slam-winning best after her tennis season ended with a second-round exit in Japan on Thursday.

The four-time Grand Slam champion lost 6-3, 7-5 to Slovakian 18-year-old Mia Pohankova, ranked 144 in the world, at the Pan Pacific Open in Tokyo.

Osaka has not won a title this year and has not reached a Grand Slam final since she returned to tennis after a 15-month break following the birth of her daughter Shai in 2023.

The 29-year-old, now ranked 18 in the world, said she was "at peace" with herself and feeling good about her game.

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"Obviously when I play a Slam, it's in my mind to challenge for it," she said. "Unfortunately I haven't been in that position yet, and I'm going to keep attempting it."

"But if you're asking me how I think my tennis is, I think it's pretty good," she added.

Osaka was appearing at the Pan Pacific Open for the first time in four years.

She came out onto the court wearing a red kimono-style gown with a long frilly train. She said she chose the color as a contrast to the blue crane-pattern outfit she wore in her opening match this week.

Osaka said she had more outfits lined up for the Tokyo crowd but "unfortunately" would not have a chance to wear them after exiting the tournament.

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Osaka is known for grabbing headlines with her custom-made outfits.

She channeled NBA fashion trailblazer Allen Iverson at the US Open and wore an audacious "jellyfish-inspired" outfit in Australia.

She also drew on her Japanese heritage with a kimono-inspired white dress at Wimbledon.

"There are so many things that go on in my mind, and it was just a creative outlet," she said. "All the other sports are allowed to do that, and tennis is more subdued and restrained."

Osaka started the match with an ace, but former junior Wimbledon champion Pohankova soon turned the tables on her.

The Slovakian, who said she grew up watching Osaka, broke her opponent three times to take the first set and kept the momentum going to win the match.

Pohankova hit seven aces, and Osaka said it was "tricky to play someone that I know nothing about."

"It is a little difficult, but I also remember when I was in her position and I was playing players that I watched on TV," Osaka said. "I guess age catches up to you eventually."

Canada's Leylah Fernandez progressed to the quarter-finals after beating number five seed Elise Mertens of Belgium 6-2, 7-6 (10/8).