Naomi Osaka, the No. 1-ranked player in the world, announced on Monday that she was splitting with her coach, Sascha Bajin, abruptly ending one of the most successful recent partnerships in women’s tennis at what seemed to be a high point.

The surprising announcement came just 16 days after Osaka won the Australian Open, her second major title under Bajin’s watch. The win vaulted her to the No. 1 ranking, a first for an Asian player on the men’s or women’s singles tours.

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

Osaka, 21, was ranked 68th when she hired Bajin in the lead-up to the 2018 season, giving him his first head coaching role. She quickly surged under his guidance, reaching the second week of a Grand Slam for the first time at the 2018 Australian Open, and then winning her first professional singles title at Indian Wells, Calif., in March.

Osaka beat Williams, her idol, for the first time in her next match in the first round of the Miami Open, and then memorably beat her again six months later in the final of the U.S. Open, a victory which took her into the top-10 for the first time.

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

Osaka’s split from Bajin is the latest on a coaches’ carousel in which success has not guaranteed longevity in player-coach partnerships. Last year’s French Open champion, Simona Halep, and the Wimbledon champion, Angelique Kerber, parted with their coaches at the end of last season, meaning that the reigning champions of the past four Grand Slam events are no longer with the coaches who helped them to those titles.

Halep, ranked third, is now working with Thierry Van Cleemput, who split with his longtime charge David Goffin at the Australian Open.

Osaka, who has not yet named a successor to Bajin, will play her first match as WTA No. 1 next week in Dubai. It will be her first time as the No. 1 seed at any WTA event.