Kerber won the Australian Open and US Open past year, as well as making the final of Wimbledon.
Japanese teenager Osaka produced an inspired performance in NY to thump Kerber 6-3, 6-1 and ensure the latter will slip out of the world's top 10 at the tournament's end.
The sixth seed held her own initially as the first six games were shared, but Osaka won nine of the next 10 with her power too much for Kerber to handle.
But then, pressure is all Kerber has known since the end of previous year, one in which she played three Grand Slam finals, winning two. The 29-year-old German didn't make it past the fourth round in a major this year, after capturing both the Australian and U.S. Open in 2016.
Kerber is also on her way out of the top 10 after losing one of the only completed matches before most of the schedule was washed out on a rainy Tuesday. Tuesday's triumph over Kerber is her most high-profile victory as she continues to climb up the world rankings.
Osaka's win was the start of an impressive day for players of color under the roof of the main court on a day where most of the matches were washed out. So the site feels really familiar to me. "I played good today, so I want to carry that on to my next match".
"This is why I come back to NY, to experience these emotions", Federer said after the match.
She was left trying to explain why, after completing the massive effort of reaching the top, it is so hard to stay there. "I mean, of course, you have always the goal for years to go there and to reach the top, but then if you are there, you actually don't know what to expect, and then it's just the process".
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Osaka used to get really nervous answering questions from the Japanese media. "That pressure is not fun but it's what you have to go through".
For Osaka, who let a 5-1 third set lead against Madison Keys slip past year on this very same court, the win is very important.
As Osaka and her older sister played as kids, they were often asked whether they were destined to be the next Williams sisters.
Showers showed up before noon on Tuesday and led to the postponements of dozens of matches.
"When it was 4-1 today I was thinking, "I really hope I don't do what I did last year" and that let me focus a lot".
Some of the blame for Kerber's slump can be attributed to the fact that the No. 1 player automatically has a target on her back; it takes a steely temperament to embrace the role.
"She said, 'No, no, we can't do that, ' " said Akatsuki Uchida, a reporter with the Japanese news organization Smash.
Dive in for all the on and off court action from Day 2 at at Flushing Meadows.