Naomi Osaka surprisingly split from Coach Sascha Bajin, a little more than two weeks after she won the Australian Open for a second consecutive Grand Slam title and moved to No. 1 in the WTA rankings.
"Hey everyone, I will no longer be working together with Sascha," Osaka posted on Twitter on Monday. "I thank him for his work and wish him all the best in the future."
Osaka's agent confirmed to The Associated Press that Osaka would no longer be working with Bajin, the 2018 WTA Coach of the Year, but said there would be no further comment.
Osaka hired Bajin before the 2018 season after he had previously worked with Serena Williams, Victoria Azarenka and Caroline Wozniacki.
Osaka withdrew from this week's Qatar Open, citing a back injury.
The 21-year-old beat Petra Kvitova in three sets in the Australian Open final on 26 January to add to her championship at the US Open last September, when she defeated Serena Williams in straight sets.
Those back-to-back victories made Osaka the first woman to win two major championships in a row since Williams picked up four straight in 2014-15.
Osaka became the youngest woman to make her debut at No 1 since Wozniacki at 20 in 2010 and the first tennis player from Asia to reach the top of the women's or men's tennis rankings.
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, 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.
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.