Aug 25 (Reuters) - Questions abound as Naomi Osaka prepares to mount a title defence at the U.S. 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. The Japanese player won the Australian Open earlier this year before opening up a conversation about mental health when she withdrew from the French Open and admitted she had struggled in the past with depression. She skipped Wimbledon but, since returning, has failed to get past the third round of the Tokyo Olympics or last week"s Western & Southern Open in Cincinnati. "Osaka obviously has been having some struggles that she"s come forward with. It"s a little unpredictable," ESPN analyst and Hall of Famer Pam Shriver told reporters on a call. "But she"s going back to a place where she"s won it twice. Usually when you have those kind of special memories, you can play some pretty good tennis," she said. Osaka defeated Williams at Flushing Meadows in 2018 for the first of her two U.S. Open titles in a final where the American clashed with the chair umpire. World number two Osaka was praised for keeping her composure throughout the incident. "I expect her to play well," Shriver said. "But I do have more questions going into this U.S. Open than I would have had the last few months been smoother for Naomi." OSAKA UPBEAT Osaka herself struck an upbeat tone after falling to Jil Teichmann in Cincinnati last week, saying the flip side of not having as many matches under her belt was that she was healthy. Osaka was hampered at last year"s U.S. Open by a left hamstring injury. "Last year, I played a lot of matches before the Open, and then I injured myself. So I don"t really want to go through that again, even though it worked out," Osaka told reporters. She said the key was to stay grounded in the moment. "I"m going into the tournament and taking it one match at a time, that"s how I play really well," she said. ESPN analyst and former world number one John McEnroe said of Osaka: "She needs to win a close one. She was double match point down against (Garbine) Muguruza at the (fourth round of the) Australian Open and pulled that out. Then she got on a roll. "I think if she"s going to do well, it"s going to have to be one of those situations," he added. The U.S. Open runs from Monday until Sept. 12.