Japanese tennis superstar Naomi Osaka announced on Friday she was splitting with her coach, her second break-up this year as she battles for form and slips down the world rankings.

On the eve of the Pan Pacific Open tournament in Osaka, western Japan, the 21-year-old announced she would no longer be working with Jermaine Jenkins, who joined her team after her triumph at the Australian Open in January.

Osaka had already caught the tennis world by surprise when she announced she was parting ways with coach, Sascha Bajin, who had overseen her stunning rise to win two grand slams — the US Open in 2018 and the Australian Open title in January.

But her results since Melbourne have been patchy. She lost in the third round of the French Open to Katerina Siniakova from the Czech Republic, ranked 41 places below her in the world rankings.

She then suffered a humiliating first-round exit at Wimbledon to Yulia Putintseva from Kazakhstan before crashing out of this month’s US Open in the last 16 to Swiss player Belinda Bencic.

Her form outside the slams has hardly been better, losing two quarter-finals during the US hard-court season that has seen her slip from the number one position in the world rankings to number four in the latest list.

The split with Jenkins came as Osaka returned home for the Pan Pacific Open where she reached the final last year.

Her main challengers in the event will be fellow top-10 players Kiki Bertens and US sensation Sloane Stephens.

Angelique Kerber, who beat her in the first round in 2017, is also in the draw for the competition that runs between September 16 and 22.