"Hey everyone, I will no longer be working together with Sascha", Osaka wrote.
She also thanked Bajin and wished him "all the best in the future".
During that period she won the prestigious Indian Wells tournament before scooping her first major in NY, while Bajin was named WTA coach of the year.
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The 21-year-old was victorious in the Australian Open last month, to add to the US Open title she won in September last year.
Osaka shared an unexpected tweet and announced her decision to end the duo's time together.
He replied to Osaka on Twitter by saying: "Thank you Naomi".
Bajin has previously worked with Serena Williams and Victoria Azarenka as a hitting partner.
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Osaka will next appear in Dubai Duty Free Tennis Championships, beginning next week.
Osaka enjoyed a remarkable rise up the world rankings after she linked up with Bajin in the 2017 off-season.
The 21-year-old beat Petra Kvitova in three sets in the Australian Open final on January 26.
By winning a second grand slam and becoming world No 1, she firmly established herself as at the forefront of the next generation of women players.
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