Former world No. 1 Naomi Osaka has announced that she's no longer working with coach Wim Fissette.

Osaka had expressed frustration with her recent results after a second-round exit at the U.S. Open just over two weeks ago.

The 26-year-old Osaka won two of her four Grand Slam titles while working with the Belgian coach. Those were the 2020 U.S. Open and the 2021 Australian Open.

It's the second time Osaka and Fissette have parted ways.

On her Instagram story late Friday, the Japanese player wrote: "4 years, 2 Slams and a whole lot of memories. Thanks Wim for being a great coach and an even greater person, wishing you all the best." She's currently ranked No. 75.

Fissette began working with Osaka shortly before the start of the 2020 season. They had split in the summer of 2022 before teaming up again last year when Osaka returned to training following maternity leave.

"In 15 years on tour, this was one of my best years working together with amazing people," Fissette wrote in an Instagram post on Saturday. "From every defeat , we worked harder and better and grew a lot. We were very close to a breakthrough."

After the U.S. Open, Osaka "decided to move forward with a new path," he added. "I accepted it knowing we close this second run with more good memories and gratitude for building her way back."

At Flushing Meadows, Osaka lost 6-3, 7-6 (5) to Karolina Muchova in the second round. Afterward, she said: "It's a little rough, because I do take these losses really personally. It's like a dramatic word, but I feel like my heart dies every time I lose."

Fissette had previously worked with players such as Victoria Azarenka, Kim Clijsters, Simona Halep and Angelique Kerber.