New York, New York - Japanese tennis star Naomi Osaka is parting ways with Belgian coach Wim Fissette, the former world number one said on Instagram on Friday.

"4 years, 2 Slams and a whole lot of memories," Osaka wrote in a post to her Instagram stories. "Thanks Wim for being a great coach and an even greater person. Wishing you all the best."

Fissette had two stints coaching Osaka, from 2019 through the summer of 2022 and again when they reunited last year as Osaka launched her return to the game after the birth of her daughter Shai in July of 2023.

Osaka won the 2020 US Open and 2021 Australian Open titles under his guidance, and also reached the finals of the 2020 Cincinnati Open and the 2022 Miami Open.

But the four-time Grand Slam champion has struggled to put together victories since she returned to the WTA Tour at Brisbane in January.

Her US Open first-round win over Jelena Ostapenko last month was her first in four years over a top-10 player – and she bowed out in the second round at Flushing Meadows to 52nd-ranked Karolina Muchova, who made it to the last four.

"It's been a little difficult because obviously I can only gauge how I'm doing by results," she said after falling to Muchova.

"I feel faster. I feel better, but I lost in the second round. So it's a little rough," added Osaka, who has fallen to 75th in the world and has reached just two quarter-finals in 16 tournaments since she returned.

Osaka said she was trying to take a more mature attitude to her defeats, but admitted to some frustration.

"I feel like I'm working way harder than I've ever worked in my life, so it needs to turn into something," she said. "Well, it doesn't need to, but I think it will."