TOKYO • Naomi Osaka shocked the tennis world on Monday after she announced she was splitting with her German coach Sascha Bajin, abruptly ending one of the most successful recent partnerships in women's tennis.

The surprising news came just 16 days after Osaka won the Australian Open, her second Grand Slam title under Bajin's watch following last year's US Open triumph.

The win in Melbourne vaulted the Japanese to the world No. 1 ranking, a first for an Asian male or female singles player.

"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," the 21-year-old wrote on Twitter.

Bajin, who became a coach after serving as the long-time hitting partner of Serena Williams, was named the WTA's Coach of the Year in 2018. He responded on Twitter to Osaka's announcement.

"Thank you Naomi," Bajin wrote.

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

The reasons for the split were unclear. Bajin did not respond to messages seeking comment, and Osaka's agent Stuart Duguid declined to comment.

The Japanese media, however, cited Osaka's management company IMG in reporting that there had been a breakdown in the player's relationship with the coach since the start of the year.

According to Japan's Nikkan Sports, some practice sessions with Bajin at the Australian Open lasted just 10 minutes and she sometimes hit without him present.

Osaka, who is scheduled to compete in Dubai next week, reportedly began sounding out potential replacements late last year.

American tennis legend Lindsay Davenport told The Tennis Channel: "It's a shocker... They started working together in December of 2017 and it's just been nothing but improvement for Osaka. That is really surprising."

The split is the latest on a coaches' carousel in which success has not guaranteed longevity in player-coach partnerships.

The 2018 French Open champion Simona Halep and Wimbledon champion Angelique Kerber both parted with their coaches after those victories, meaning that the reigning champions of the past four Grand Slam events are no longer with the coaches who helped them to those titles.

NYTIMES, AGENCE FRANCE-PRESSE