TOKYO, July 25 (Reuters) - 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 lacklustre, 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 Centre 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 on hard courts - the same surface used at the Ariake Tennis Park. Osaka said she was feeling refreshed having been on an extended break since May to focus on her mental health. read more The world number two was also unbothered by the sweltering heat that has troubled other players in Tokyo. "It feels fine for me... I love being here, and I actually really like the weather," she said. "STIFLING CONDITIONS" On Saturday, the top two men"s players, Novak Djokovic and Daniil Medvedev, had called on organisers to move matches to the late afternoon to save players from the stifling conditions. read more Australian Barty, a twice Grand Slam champion fresh off her victory at Wimbledon, was dumped out of the women"s singles after a stunning 6-4 6-3 loss to 48th-ranked Sara Sorribes Tormo. Barty never looked comfortable against the Spaniard, who took advantage of a string of errors off the 25-year-old"s racket to set up a meeting with France"s Fiona Ferro. "I never really felt comfortable out there and wasn’t able to play the match on my terms," Barty said. "The key to my game is serving well and I wasn’t able to do that today. I was a bit erratic and made too many errors." Her compatriot Max Purcell was more fortunate, upsetting ninth seed Felix Auger-Aliassime of Canada shortly after he was announced as a replacement for British player Andy Murray. Murray, who had been aiming to become the first player to win three Olympic singles gold medals, pulled out of the tournament hours before the start of his opening match due to a thigh strain. Meanwhile, French Open finalist Stefanos Tsitsipas survived a tight three-setter against Germany"s Philip Kohlschreiber, coming back from a break down in the final set to win 6-3 3-6 6-3. The Greek third seed will next play Frances Tiafoe, who beat him in the first round at Wimbledon last month. Osaka"s Japanese teammate Kei Nishikori, who won the bronze medal at Rio 2016, also advanced to the second round after beating Andrey Rublev of the Russian Olympic Committee. read more