TOKYO—Japan’s Naomi Osaka powered through her opening match at the Tokyo Games on Sunday as world number one Ash Barty stumbled out of the tournament after a lackluster, error-strewn performance. Osaka, who is aiming to become only the fourth singles player since 1988 to win Olympic gold on home soil, admitted to having the jitters but showed no signs of pressure in dismantling China’s Zheng Saisai 6–1, 6–4 on Center Court. “I felt really nervous being in Japan and playing here for the first time in maybe two years, and for it to be my first Olympics,” Osaka told reporters. “It was definitely really nerve-wracking. But I am glad I was able to win. She is a very tough opponent.” The four-times Grand Slam champion, who lit the Olympic cauldron on Friday, is widely seen as one of Japan’s biggest medal contenders with all four of her major titles coming …