Naomi Osaka, winner of the past two grand slam tournaments, has parted ways with her coach Sascha Bajin.

Osaka became world number one when she won the Australian Open last month, after previously beating Serena Williams in the U.S. Open final in September.

“I will no longer be working together with Sascha,” tweeted the U.S.-based Japanese 21-year-old.

“I thank him for his work and wish him all the best in the future.”

Bajin replied to Osaka with a tweet of his own.

“Thank you Naomi. I wish you nothing but the best as well. What a ride that was. Thank you for letting me be part of this.”

Osaka hired Bajin in 2017 and enjoyed a meteoric rise during the time they worked together.