Japan's Naomi Osaka announced Monday that she has parted ways with coach Sascha Bajin, two weeks after becoming the first player from Asia to top either the men's or women's singles rankings.

"Hey everyone, I will no longer be working together with Sascha. I thank him for his work and wish him all the best in the future," Osaka tweeted.

Bajin, who was named 2018 WTA coach of the year, responded on Twitter, "I wish you nothing but the best as well. What a ride that was. Thank you for letting me be part of this."

Osaka, who won her second straight Grand Slam at the Australian Open last month, is in the process of selecting a new coach, according to sources close to the matter.

Since Bajin had no prior experience coaching a top-ranked player, Osaka's team started discussions last year about hiring somebody else to take over the job, a source said.

She is looking for somebody who can coach the tactics and techniques required to compete at the highest level, the source said.

A former hitting partner for Serena Williams, Bajin has been credited with helping Osaka improve the mental aspect of her game during her breakthrough 2018 season.

Under the German's tutelage, Osaka became just the 10th woman in history to win the U.S. Open and Australian Open back-to-back, and the first-ever Asian world No. 1 following her triumph at the Australian Open, according to the Women's Tennis Association.

Osaka's victory at the Australian Open ended Romanian Simona Halep's 48-week reign atop the WTA rankings. She claimed her maiden Grand Slam title at the U.S. Open last September over Williams, her childhood idol.

The 21-year-old is sitting out the ongoing Qatar Open in Doha due to back pain, but is expected to show at the Dubai Tennis Championships in the United Arab Emirates starting Sunday.