The 21-year-old Osaka, who won at Melbourne Park last month to add to her US Open crown from last year, announced on Monday that her successful partnership with Bajin had come to an end.
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.
She became the first Asian to hold the top ranking after downing Czech Petra Kvitova in a tight final in Australia after beating Serena Williams in a memorable US Open final previous year in which she won many fans for her grace under pressure.More news: Saudi named in report on Khashoggi murder becomes UAE envoy
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He responded to Osaka's tweet by replying: 'Thank you Naomi I wish you nothing but the best as well.
Osaka's Twitter feed lit up in response to her announcement, with several memes expressing widespread disbelief - including a shrugging Barack Obama, a confused Joey from American sitcom "Friends" and a Minions character screaming "WHAAAAT?" He thanked her and wrote: "I wish you nothing but the best as well".
Osaka hired Bajin in 2017 and enjoyed a meteoric rise during the time they worked together.
Osaka withdrew from this week's Qatar Open, citing a back injury.