Former world number one Naomi Osaka says she "hasn't had fun playing tennis" since winning the Australian Open.

Osaka, 21, beat Czech Petra Kvitova in a thrilling final in January to seal back-to-back Grand Slams, topping the world rankings.

But she has been hampered by injuries and struggled with poor form since.

"The last few months have been really rough for me tennis-wise," Osaka wrote on Instagram.

"Thankfully I am surrounded by people I love and who love me back.

"Whenever things go wrong I blame myself 100%. I have a tendency to shut down because I don't want to burden anyone with my thoughts or problems, but they taught me to trust them and not take everything on by myself.

"Unexpectedly though the worst months of my life have also had some of the best moments because I've met new people and been able to do things that I have never even considered doing before.

"That being said I can honestly reflect and say I probably haven't had fun playing tennis since Australia and I'm finally coming to terms with that while relearning that fun feeling."

Osaka parted ways with coach Sascha Bajin after the Australian Open, withdrew from the Stuttgart Open and Italian Open through injury, then suffered defeats at the French Open and Wimbledon.

"I have put so much weight on the results of my matches instead of learning from them, which is what I 'normally' do," she wrote.

"I've learned a lot about myself and feel I grew so much as a person these past years so I'm really excited what the future looks like.

"See you in the US swing."

Osaka, who lost her world number one ranking in June, aims to defend her US Open title at Flushing Meadows, starting 26 August.