The day after the historic victory, the 20 year old said in an exclusive interview with NHK that it was one of the best matches of her life. She said she felt like all of the hard work she had done in her life was for that moment, and being able to play against Serena Williams was really special.
Asked about Williams' violations during the match, Osaka said she hadn't been aware of what was going on and that she first learned about it when the referee said the score was 5-3. She said she was completely focused on the match as that was necessary when competing against a player like Williams.
Behind her historic victory was her 8-month long training with "Team Naomi." Together with a team of professionals, she succeeded in building up her mental and physical strength as well as techniques.
Osaka began training under her coach Sascha Bajin last November. The German coach is the former hitting partner of top players including Serena Williams.
Under Bajin, Osaka's weight dropped by about 7 kilograms in the off-season, and she began pursuing a new style rather than relying on power. Bajin taught her to wait for a chance to attack in a rally rather than hit hard every time, to reduce mistakes.
He also taught her to take control with a mix of various serves, instead of scoring with a fast serve of nearly 200 kilometers per hour. He also encouraged her to feel positive and stay mentally stable during matches.
Osaka built up physical strength under her trainer Abdul Sillah, the former trainer of Williams. She managed to strengthen her trunk and the lower half of her body through a program combining running, muscle training and bicycling.
She also gained suppleness around her hip joints under another trainer, Natsuko Mogi.
The trainers say one of Osaka's greatest talents is her willingness to heed and implement their advice. They were happy with her steady progress this season.
Their efforts bore fruit at the US open. Osaka found success in her mix of serves. She dipped deep to hit sharp strokes at key points.
As she trained under Sillah and Mogi, Osaka learned to lower her weight and hit in a stable position. That helped her reduce mistakes at key points in strokes. She won 3 consecutive matches from the first round in straight sets.
Something unusual happened after practice just ahead of her 4th round match in the US Open. During practice, Osaka's movements and expression became tense amid the growing possibility she would advance to the quarterfinals.
In an attempt to relax Osaka, her coach asked her to hit the balls she used during the practice to the stands as a gift to her fans. Her coach and trainer also brought tennis balls to the stands. Osaka broke into a smile for the first time as fans cheered for her.
In her 4th round match that followed, Osaka lost the first set, but she persevered to capture a victory. She went on to advance to the quarterfinals and the semi-finals, winning in straight sets.
Osaka's efforts to build up her physical strength paid off in the match against former world tennis champion Williams. With both players being power hitters, the match was set to be a series of long rallies filled with tough strokes. But Osaka's ability to make Williams run left to right became the key to her victory.
Williams lost her balance and made a series of errors as she struggled to return Osaka's powerful deep shots that came just inside the baseline. In comparison, Osaka kept her posture and was able to return shots with high precision.
In the 2nd set, after Williams was given penalties, the crowd began to boo and the atmosphere became unusually tense. But Osaka was able to maintain her concentration until the very end. The mental training program she underwent under the supervision of coach Bajin paid off.
Osaka was born in Japan to a Japanese mother and Haitian father. She was raised in the United States.
Many people across Japan have found her feat joyful and encouraging at a time the country's suffering from the aftermath of a deadly earthquake and typhoon. In Osaka, the tennis star's hometown, a local newspaper published an extra edition to report her victory. A local said she was astonished to hear a Japanese player had won the Grand Slam.
Employees of a major Japanese food manufacturer with which Osaka has a sponsorship deal gathered to cheer for her before a big screen set up at their office in Tokyo. One employee said in excitement after the match that he was moved to be able to see the historic moment and said he hoped Osaka would keep up her good work for Japan and the rest of the world.
Osaka's 73-year-old grandfather watched the match at his home in Nemuro, Hokkaido. He told reporters he was overjoyed and said he had never expected to receive such a great gift at his age. He also said he hopes she will continue to perform well without getting injured.
Other Japanese athletes are also applauding her. US Open semifinalist Kei Nishikori congratulated her on Twitter. Major leaguer Shohei Ohtani said he was moved that Osaka was able to achieve what Japanese tennis players had attempted for many years, and that she was an inspiration.
But Osaka doesn't have much time to bask in her victory. She says her focus is now on her next tournament starting September 17th in Tokyo. Asked if she thinks about the 2020 Tokyo Olympics, she replied that she does, but that doing so too much will give her stress, so she would rather focus on doing her best in her imminent matches.