Defending U.S. Open champion Angelique Kerber is out of this year's tournament after a stunning first-round loss.
While Kerber will be seeking answers after her early exit, top-seed Karolina Pliskova had no problems moving into the second round with a straight-sets win over Magda Linette.
Osaka may downed defending champion Angelique Kerber before facing a more time-consuming task - tracking down her family movements. Federer went up 3-1, then got broken while serving for the match at 5-3.
"I was thinking about it on the court: 'If I win, what would I do if I won?" "I have not been really lucky with the weather for last two tournaments, Toronto and Cincinnati", Pliskova said with a laugh.
Angelique Kerber insisted she can return to the level that made her a two-time grand slam champion and world number one after a disappointing season hit a new low.
For Kerber, it was a fitting end to the major calendar - she followed 2016 triumphs at the Australian and U.S. Open by failing to reach even the quarterfinals of a major this year and thoroughly frittering away her No. 1 ranking.
The German will fall out of the Top 10 after the U.S. Open.
She was left trying to explain why, after completing the massive effort of reaching the top, it is so hard to stay there.
"It was not my day; completely not my day today", the world No. 6 lamented. Then I didn't really know what to do, should I warm up or should I go back to hotel?
"It took a while to get used to it".
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I, for one, was saying they should be playing against Ireland, Afghanistan and Zimbabwe and they bounced back with a performance like that.
"But for me it was for sure not the best day and not the best match".
As Osaka and her older sister played as kids, they were often asked whether they were destined to be the next Williams sisters.
She said she had been practicing well and was confident, despite some minor nagging injuries she had bothered her throughout the season.
"I think when you see the end result, you think it just clicked, but it is a process, and it's from losing some matches that are painful".
Kerber's drop in 2017 has been almost as remarkable in her rise in 2016.
"I tried not to think how good she was while I was playing her", the Japanese teenager said.
Osaka was less certain about her emotions after the victory.
"She said, 'No, no, we can't do that, ' " said Akatsuki Uchida, a reporter with the Japanese news organization Smash.
"I felt the same type of nerves come up, 4-1 in this match", Osaka said afterward, "So I wanted to tell myself just to keep playing how I was playing and not let the nerves get over me as much as a year ago". "I was really glad when she made an error". I don't really know, it's, like, nostalgic every time I come.