NEW YORK: Questions abound as Naomi Osaka (pix) prepares to mount a title defence at the US Open following a series of lacklustre performances since she returned to the game after stepping away to focus on her mental health.

Osaka shot to global fame on the back of her four Grand Slam titles and support of racial justice causes, with many seeing the hard-hitting 23-year-old as the heir apparent to Serena Williams.