Naomi Osaka has split with coach Sascha Bajin, the world number one announced on Monday.
Bajin became the first recipient of the WTA Coach of the Year award after helping Osaka take her game to new heights in a breakthrough 2018.
The Japanese claimed the first WTA title of her career at Indian Wells in March, defeating Maria Sharapova, Karolina Pliskova and Simona Halep en route to the title.
Osaka secured her first grand slam title at the US Open, beating Serena Williams in a final that was overshadowed by the 23-time major winner's argument with umpire Carlos Ramos.
The 21-year-old added the Australian Open to her growing list of honours in January and consequently topped the world rankings for the first time in her career.
Despite her success alongside Bajin, Osaka confirmed the pair have parted ways on Twitter.
"I will no longer be working together with Sascha," Osaka wrote. "I thank him for his work and wish him all the best in the future."
Osaka, who withdrew from this week's Qatar Open due to a back injury, has not yet announced a replacement for Bajin.
He replied to Osaka on Twitter by saying: "Thank you Naomi. I wish you nothing but the best as well. What a ride it was. Thank you for letting me be part of this."